HS: MutationObserver mock + on mutation dispatch (+7 tests)

Parser: parse-on-feat now consumes `of FILTER` after `mutation` event-name,
where FILTER is `attributes`/`childList`/`characterData` ident or `@a [or @b]*`
attr-token chain. Emits :of-filter dict on parts. Compiler: scan-on threads
of-filter-info; mutation event-name emits `(do (hs-on …) (hs-on-mutation-attach!
TARGET MODE ATTRS))`. Runtime: hs-on-mutation-attach! constructs a real
MutationObserver with config matched to filter and dispatches "mutation" event
with records detail. Runner: HsMutationObserver mock with global registry;
prototype hooks on El.setAttribute/appendChild/removeChild/_setInnerHTML fire
matching observers synchronously, with __hsMutationActive guard preventing
recursion. Generator: dropped 7 mutation tests from skip-list, added
evaluate(setAttribute) and evaluate(appendChild) body patterns.

hs-upstream-on: 36/70 → 43/70. Smoke 0-195 unchanged at 170/195.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/hyperscript/compiler.sx

@@ -164,7 +164,8 @@
                   every?
                   catch-info
                   finally-info
-                  having-info)
+                  having-info
+                  of-filter-info)
                 (cond
                   ((<= (len items) 1)
                     (let
@@ -185,23 +186,44 @@
                                     ((handler (let ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false))))) (list (quote fn) (list (quote event)) (if (uses-the-result? wrapped-body) (list (quote let) (list (list (quote the-result) nil)) wrapped-body) wrapped-body)))))
                                     (let
                                       ((on-call (if every? (list (quote hs-on-every) target event-name handler) (list (quote hs-on) target event-name handler))))
-                                      (if
+                                      (cond
-                                        (= event-name "intersection")
+                                        ((= event-name "mutation")
-                                        (list
-                                          (quote do)
-                                          on-call
                                           (list
-                                            (quote hs-on-intersection-attach!)
+                                            (quote do)
-                                            target
+                                            on-call
-                                            (if
+                                            (list
-                                              having-info
+                                              (quote hs-on-mutation-attach!)
-                                              (get having-info "margin")
+                                              target
-                                              nil)
+                                              (if
-                                            (if
+                                                of-filter-info
-                                              having-info
+                                                (get of-filter-info "type")
-                                              (get having-info "threshold")
+                                                "any")
-                                              nil)))
+                                              (if
-                                        on-call)))))))))))
+                                                of-filter-info
+                                                (let
+                                                  ((a (get of-filter-info "attrs")))
+                                                  (if
+                                                    a
+                                                    (cons (quote list) a)
+                                                    nil))
+                                                nil))))
+                                        ((= event-name "intersection")
+                                          (list
+                                            (quote do)
+                                            on-call
+                                            (list
+                                              (quote
+                                                hs-on-intersection-attach!)
+                                              target
+                                              (if
+                                                having-info
+                                                (get having-info "margin")
+                                                nil)
+                                              (if
+                                                having-info
+                                                (get having-info "threshold")
+                                                nil))))
+                                        (true on-call))))))))))))
                   ((= (first items) :from)
                     (scan-on
                       (rest (rest items))
@@ -210,7 +232,8 @@
                       every?
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :filter)
                     (scan-on
                       (rest (rest items))
@@ -219,7 +242,8 @@
                       every?
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :every)
                     (scan-on
                       (rest (rest items))
@@ -228,7 +252,8 @@
                       true
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :catch)
                     (scan-on
                       (rest (rest items))
@@ -237,7 +262,8 @@
                       every?
                       (nth items 1)
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :finally)
                     (scan-on
                       (rest (rest items))
@@ -246,7 +272,8 @@
                       every?
                       catch-info
                       (nth items 1)
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :having)
                     (scan-on
                       (rest (rest items))
@@ -255,6 +282,17 @@
                       every?
                       catch-info
                       finally-info
+                      (nth items 1)
+                      of-filter-info))
+                  ((= (first items) :of-filter)
+                    (scan-on
+                      (rest (rest items))
+                      source
+                      filter
+                      every?
+                      catch-info
+                      finally-info
+                      having-info
                       (nth items 1)))
                   (true
                     (scan-on
@@ -264,8 +302,9 @@
                       every?
                       catch-info
                       finally-info
-                      having-info)))))
+                      having-info
-            (scan-on (rest parts) nil nil false nil nil nil)))))
+                      of-filter-info)))))
+            (scan-on (rest parts) nil nil false nil nil nil nil)))))
     (define
       emit-send
       (fn

lib/hyperscript/parser.sx

@@ -2605,59 +2605,66 @@
             (let
               ((event-name (parse-compound-event-name)))
               (let
-                ((flt (if (= (tp-type) "bracket-open") (do (adv!) (let ((f (parse-expr))) (if (= (tp-type) "bracket-close") (adv!) nil) f)) nil)))
+                ((of-filter (when (and (= event-name "mutation") (match-kw "of")) (cond ((and (= (tp-type) "ident") (or (= (tp-val) "attributes") (= (tp-val) "childList") (= (tp-val) "characterData"))) (let ((nm (tp-val))) (do (adv!) (dict "type" nm)))) ((= (tp-type) "attr") (let ((attrs (list (tp-val)))) (do (adv!) (define collect-or! (fn () (when (match-kw "or") (cond ((= (tp-type) "attr") (do (set! attrs (append attrs (list (tp-val)))) (adv!) (collect-or!))) (true (set! p (- p 1))))))) (collect-or!) (dict "type" "attrs" "attrs" attrs)))) (true nil)))))
                 (let
-                  ((source (if (match-kw "from") (parse-expr) nil)))
+                  ((flt (if (= (tp-type) "bracket-open") (do (adv!) (let ((f (parse-expr))) (if (= (tp-type) "bracket-close") (adv!) nil) f)) nil)))
                   (let
-                    ((h-margin nil) (h-threshold nil))
+                    ((source (if (match-kw "from") (parse-expr) nil)))
-                    (define
-                      consume-having!
-                      (fn
-                        ()
-                        (cond
-                          ((and (= (tp-type) "ident") (= (tp-val) "having"))
-                            (do
-                              (adv!)
-                              (cond
-                                ((and (= (tp-type) "ident") (= (tp-val) "margin"))
-                                  (do
-                                    (adv!)
-                                    (set! h-margin (parse-expr))
-                                    (consume-having!)))
-                                ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
-                                  (do
-                                    (adv!)
-                                    (set! h-threshold (parse-expr))
-                                    (consume-having!)))
-                                (true nil))))
-                          (true nil))))
-                    (consume-having!)
                     (let
-                      ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
+                      ((h-margin nil) (h-threshold nil))
+                      (define
+                        consume-having!
+                        (fn
+                          ()
+                          (cond
+                            ((and (= (tp-type) "ident") (= (tp-val) "having"))
+                              (do
+                                (adv!)
+                                (cond
+                                  ((and (= (tp-type) "ident") (= (tp-val) "margin"))
+                                    (do
+                                      (adv!)
+                                      (set! h-margin (parse-expr))
+                                      (consume-having!)))
+                                  ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
+                                    (do
+                                      (adv!)
+                                      (set! h-threshold (parse-expr))
+                                      (consume-having!)))
+                                  (true nil))))
+                            (true nil))))
+                      (consume-having!)
                       (let
-                        ((body (parse-cmd-list)))
+                        ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
                         (let
-                          ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                          ((body (parse-cmd-list)))
-                            (finally-clause
-                              (if (match-kw "finally") (parse-cmd-list) nil)))
-                          (match-kw "end")
                           (let
-                            ((parts (list (quote on) event-name)))
+                            ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                              (finally-clause
+                                (if
+                                  (match-kw "finally")
+                                  (parse-cmd-list)
+                                  nil)))
+                            (match-kw "end")
                             (let
-                              ((parts (if every? (append parts (list :every true)) parts)))
+                              ((parts (list (quote on) event-name)))
                               (let
-                                ((parts (if flt (append parts (list :filter flt)) parts)))
+                                ((parts (if every? (append parts (list :every true)) parts)))
                                 (let
-                                  ((parts (if source (append parts (list :from source)) parts)))
+                                  ((parts (if flt (append parts (list :filter flt)) parts)))
                                   (let
-                                    ((parts (if having (append parts (list :having having)) parts)))
+                                    ((parts (if source (append parts (list :from source)) parts)))
                                     (let
-                                      ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                      ((parts (if of-filter (append parts (list :of-filter of-filter)) parts)))
                                       (let
-                                        ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                        ((parts (if having (append parts (list :having having)) parts)))
                                         (let
-                                          ((parts (append parts (list body))))
+                                          ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
-                                          parts))))))))))))))))))
+                                          (let
+                                            ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                            (let
+                                              ((parts (append parts (list body))))
+                                              parts))))))))))))))))))))
       (define
         parse-init-feat
         (fn

lib/hyperscript/runtime.sx

@@ -82,14 +82,36 @@
           observer)))))
 
 ;; Wait for CSS transitions/animations to settle on an element.
-(define hs-init (fn (thunk) (thunk)))
+(define
+  hs-on-mutation-attach!
+  (fn
+    (target mode attr-list)
+    (let
+      ((cfg-attributes (or (= mode "any") (= mode "attributes") (= mode "attrs")))
+        (cfg-childList (or (= mode "any") (= mode "childList")))
+        (cfg-characterData (or (= mode "any") (= mode "characterData"))))
+      (let
+        ((opts (dict "attributes" cfg-attributes "childList" cfg-childList "characterData" cfg-characterData "subtree" true)))
+        (when
+          (and (= mode "attrs") attr-list)
+          (dict-set! opts "attributeFilter" attr-list))
+        (let
+          ((cb (fn (records observer) (dom-dispatch target "mutation" (dict "records" records)))))
+          (let
+            ((observer (host-new "MutationObserver" cb)))
+            (host-call observer "observe" target opts)
+            observer))))))
 
 ;; ── Class manipulation ──────────────────────────────────────────
 
 ;; Toggle a single class on an element.
-(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+(define hs-init (fn (thunk) (thunk)))
 
 ;; Toggle between two classes — exactly one is active at a time.
+(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+
+;; Take a class from siblings — add to target, remove from others.
+;; (hs-take! target cls) — like radio button class behavior
 (begin
   (define
     hs-wait-for
@@ -102,21 +124,20 @@
       (target event-name timeout-ms)
       (perform (list (quote io-wait-event) target event-name timeout-ms)))))
 
-;; Take a class from siblings — add to target, remove from others.
-;; (hs-take! target cls) — like radio button class behavior
-(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-
 ;; ── DOM insertion ───────────────────────────────────────────────
 
 ;; Put content at a position relative to a target.
 ;; pos: "into" | "before" | "after"
-(define
+(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-  hs-toggle-class!
-  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
 
 ;; ── Navigation / traversal ──────────────────────────────────────
 
 ;; Navigate to a URL.
+(define
+  hs-toggle-class!
+  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+
+;; Find next sibling matching a selector (or any sibling).
 (define
   hs-toggle-between!
   (fn
@@ -126,7 +147,7 @@
       (do (dom-remove-class target cls1) (dom-add-class target cls2))
       (do (dom-remove-class target cls2) (dom-add-class target cls1)))))
 
-;; Find next sibling matching a selector (or any sibling).
+;; Find previous sibling matching a selector.
 (define
   hs-toggle-style!
   (fn
@@ -150,7 +171,7 @@
             (dom-set-style target prop "hidden")
             (dom-set-style target prop "")))))))
 
-;; Find previous sibling matching a selector.
+;; First element matching selector within a scope.
 (define
   hs-toggle-style-between!
   (fn
@@ -162,7 +183,7 @@
         (dom-set-style target prop val2)
         (dom-set-style target prop val1)))))
 
-;; First element matching selector within a scope.
+;; Last element matching selector.
 (define
   hs-toggle-style-cycle!
   (fn
@@ -183,7 +204,7 @@
             (true (find-next (rest remaining))))))
       (dom-set-style target prop (find-next vals)))))
 
-;; Last element matching selector.
+;; First/last within a specific scope.
 (define
   hs-take!
   (fn
@@ -223,7 +244,6 @@
               (dom-set-attr target name attr-val)
               (dom-set-attr target name ""))))))))
 
-;; First/last within a specific scope.
 (begin
   (define
     hs-element?
@@ -335,6 +355,9 @@
                 (dom-insert-adjacent-html target "beforeend" value)
                 (hs-boot-subtree! target)))))))))
 
+;; ── Iteration ───────────────────────────────────────────────────
+
+;; Repeat a thunk N times.
 (define
   hs-add-to!
   (fn
@@ -347,9 +370,7 @@
           (append target (list value))))
       (true (do (host-call target "push" value) target)))))
 
-;; ── Iteration ───────────────────────────────────────────────────
+;; Repeat forever (until break — relies on exception/continuation).
-
-;; Repeat a thunk N times.
 (define
   hs-remove-from!
   (fn
@@ -359,7 +380,10 @@
       (filter (fn (x) (not (= x value))) target)
       (host-call target "splice" (host-call target "indexOf" value) 1))))
 
-;; Repeat forever (until break — relies on exception/continuation).
+;; ── Fetch ───────────────────────────────────────────────────────
+
+;; Fetch a URL, parse response according to format.
+;; (hs-fetch url format) — format is "json" | "text" | "html"
 (define
   hs-splice-at!
   (fn
@@ -383,10 +407,10 @@
               (host-call target "splice" i 1))))
         target))))
 
-;; ── Fetch ───────────────────────────────────────────────────────
+;; ── Type coercion ───────────────────────────────────────────────
 
-;; Fetch a URL, parse response according to format.
+;; Coerce a value to a type by name.
-;; (hs-fetch url format) — format is "json" | "text" | "html"
+;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
 (define
   hs-index
   (fn
@@ -398,10 +422,10 @@
       ((string? obj) (nth obj key))
       (true (host-get obj key)))))
 
-;; ── Type coercion ───────────────────────────────────────────────
+;; ── Object creation ─────────────────────────────────────────────
 
-;; Coerce a value to a type by name.
+;; Make a new object of a given type.
-;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
+;; (hs-make type-name) — creates empty object/collection
 (define
   hs-put-at!
   (fn
@@ -423,10 +447,11 @@
                 ((= pos "start") (host-call target "unshift" value)))
               target)))))))
 
-;; ── Object creation ─────────────────────────────────────────────
+;; ── Behavior installation ───────────────────────────────────────
 
-;; Make a new object of a given type.
+;; Install a behavior on an element.
-;; (hs-make type-name) — creates empty object/collection
+;; A behavior is a function that takes (me ...params) and sets up features.
+;; (hs-install behavior-fn me ...args)
 (define
   hs-dict-without
   (fn
@@ -447,27 +472,27 @@
           (host-call (host-global "Reflect") "deleteProperty" out key)
           out)))))
 
-;; ── Behavior installation ───────────────────────────────────────
+;; ── Measurement ─────────────────────────────────────────────────
 
-;; Install a behavior on an element.
+;; Measure an element's bounding rect, store as local variables.
-;; A behavior is a function that takes (me ...params) and sets up features.
+;; Returns a dict with x, y, width, height, top, left, right, bottom.
-;; (hs-install behavior-fn me ...args)
 (define
   hs-set-on!
   (fn
     (props target)
     (for-each (fn (k) (host-set! target k (get props k))) (keys props))))
 
-;; ── Measurement ─────────────────────────────────────────────────
-
-;; Measure an element's bounding rect, store as local variables.
-;; Returns a dict with x, y, width, height, top, left, right, bottom.
-(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
-
 ;; Return the current text selection as a string.  In the browser this is
 ;; `window.getSelection().toString()`.  In the mock test runner, a test
 ;; setup stashes the desired selection text at `window.__test_selection`
 ;; and the fallback path returns that so tests can assert on the result.
+(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
+
+
+;; ── Transition ──────────────────────────────────────────────────
+
+;; Transition a CSS property to a value, optionally with duration.
+;; (hs-transition target prop value duration)
 (define
   hs-ask
   (fn
@@ -476,11 +501,6 @@
       ((w (host-global "window")))
       (if w (host-call w "prompt" msg) nil))))
 
-
-;; ── Transition ──────────────────────────────────────────────────
-
-;; Transition a CSS property to a value, optionally with duration.
-;; (hs-transition target prop value duration)
 (define
   hs-answer
   (fn
@@ -634,6 +654,10 @@
   hs-query-all
   (fn (sel) (host-call (dom-body) "querySelectorAll" sel)))
 
+
+
+
+
 (define
   hs-query-all-in
   (fn
@@ -643,25 +667,21 @@
       (hs-query-all sel)
       (host-call target "querySelectorAll" sel))))
 
-
-
-
-
 (define
   hs-list-set
   (fn
     (lst idx val)
     (append (take lst idx) (cons val (drop lst (+ idx 1))))))
-
-(define
-  hs-to-number
-  (fn (v) (if (number? v) v (or (parse-number (str v)) 0))))
 ;; ── Sandbox/test runtime additions ──────────────────────────────
 ;; Property access — dot notation and .length
+(define
+  hs-to-number
+  (fn (v) (if (number? v) v (or (parse-number (str v)) 0))))
+;; DOM query stub — sandbox returns empty list
 (define
   hs-query-first
   (fn (sel) (host-call (host-global "document") "querySelector" sel)))
-;; DOM query stub — sandbox returns empty list
+;; Method dispatch — obj.method(args)
 (define
   hs-query-last
   (fn
@@ -669,11 +689,11 @@
     (let
       ((all (dom-query-all (dom-body) sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-;; Method dispatch — obj.method(args)
-(define hs-first (fn (scope sel) (dom-query-all scope sel)))
 
 ;; ── 0.9.90 features ─────────────────────────────────────────────
 ;; beep! — debug logging, returns value unchanged
+(define hs-first (fn (scope sel) (dom-query-all scope sel)))
+;; Property-based is — check obj.key truthiness
 (define
   hs-last
   (fn
@@ -681,7 +701,7 @@
     (let
       ((all (dom-query-all scope sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-;; Property-based is — check obj.key truthiness
+;; Array slicing (inclusive both ends)
 (define
   hs-repeat-times
   (fn
@@ -699,7 +719,7 @@
               ((= signal "hs-continue") (do-repeat (+ i 1)))
               (true (do-repeat (+ i 1))))))))
     (do-repeat 0)))
-;; Array slicing (inclusive both ends)
+;; Collection: sorted by
 (define
   hs-repeat-forever
   (fn
@@ -715,7 +735,7 @@
             ((= signal "hs-continue") (do-forever))
             (true (do-forever))))))
     (do-forever)))
-;; Collection: sorted by
+;; Collection: sorted by descending
 (define
   hs-repeat-while
   (fn
@@ -728,7 +748,7 @@
           ((= signal "hs-break") nil)
           ((= signal "hs-continue") (hs-repeat-while cond-fn thunk))
           (true (hs-repeat-while cond-fn thunk)))))))
-;; Collection: sorted by descending
+;; Collection: split by
 (define
   hs-repeat-until
   (fn
@@ -740,7 +760,7 @@
         ((= signal "hs-continue")
           (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))
         (true (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))))))
-;; Collection: split by
+;; Collection: joined by
 (define
   hs-for-each
   (fn
@@ -760,7 +780,7 @@
                 ((= signal "hs-continue") (do-loop (rest remaining)))
                 (true (do-loop (rest remaining))))))))
       (do-loop items))))
-;; Collection: joined by
+
 (begin
   (define
     hs-append

shared/static/wasm/sx/hs-compiler.sx

@@ -164,7 +164,8 @@
                   every?
                   catch-info
                   finally-info
-                  having-info)
+                  having-info
+                  of-filter-info)
                 (cond
                   ((<= (len items) 1)
                     (let
@@ -185,23 +186,44 @@
                                     ((handler (let ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false))))) (list (quote fn) (list (quote event)) (if (uses-the-result? wrapped-body) (list (quote let) (list (list (quote the-result) nil)) wrapped-body) wrapped-body)))))
                                     (let
                                       ((on-call (if every? (list (quote hs-on-every) target event-name handler) (list (quote hs-on) target event-name handler))))
-                                      (if
+                                      (cond
-                                        (= event-name "intersection")
+                                        ((= event-name "mutation")
-                                        (list
-                                          (quote do)
-                                          on-call
                                           (list
-                                            (quote hs-on-intersection-attach!)
+                                            (quote do)
-                                            target
+                                            on-call
-                                            (if
+                                            (list
-                                              having-info
+                                              (quote hs-on-mutation-attach!)
-                                              (get having-info "margin")
+                                              target
-                                              nil)
+                                              (if
-                                            (if
+                                                of-filter-info
-                                              having-info
+                                                (get of-filter-info "type")
-                                              (get having-info "threshold")
+                                                "any")
-                                              nil)))
+                                              (if
-                                        on-call)))))))))))
+                                                of-filter-info
+                                                (let
+                                                  ((a (get of-filter-info "attrs")))
+                                                  (if
+                                                    a
+                                                    (cons (quote list) a)
+                                                    nil))
+                                                nil))))
+                                        ((= event-name "intersection")
+                                          (list
+                                            (quote do)
+                                            on-call
+                                            (list
+                                              (quote
+                                                hs-on-intersection-attach!)
+                                              target
+                                              (if
+                                                having-info
+                                                (get having-info "margin")
+                                                nil)
+                                              (if
+                                                having-info
+                                                (get having-info "threshold")
+                                                nil))))
+                                        (true on-call))))))))))))
                   ((= (first items) :from)
                     (scan-on
                       (rest (rest items))
@@ -210,7 +232,8 @@
                       every?
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :filter)
                     (scan-on
                       (rest (rest items))
@@ -219,7 +242,8 @@
                       every?
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :every)
                     (scan-on
                       (rest (rest items))
@@ -228,7 +252,8 @@
                       true
                       catch-info
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :catch)
                     (scan-on
                       (rest (rest items))
@@ -237,7 +262,8 @@
                       every?
                       (nth items 1)
                       finally-info
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :finally)
                     (scan-on
                       (rest (rest items))
@@ -246,7 +272,8 @@
                       every?
                       catch-info
                       (nth items 1)
-                      having-info))
+                      having-info
+                      of-filter-info))
                   ((= (first items) :having)
                     (scan-on
                       (rest (rest items))
@@ -255,6 +282,17 @@
                       every?
                       catch-info
                       finally-info
+                      (nth items 1)
+                      of-filter-info))
+                  ((= (first items) :of-filter)
+                    (scan-on
+                      (rest (rest items))
+                      source
+                      filter
+                      every?
+                      catch-info
+                      finally-info
+                      having-info
                       (nth items 1)))
                   (true
                     (scan-on
@@ -264,8 +302,9 @@
                       every?
                       catch-info
                       finally-info
-                      having-info)))))
+                      having-info
-            (scan-on (rest parts) nil nil false nil nil nil)))))
+                      of-filter-info)))))
+            (scan-on (rest parts) nil nil false nil nil nil nil)))))
     (define
       emit-send
       (fn

shared/static/wasm/sx/hs-parser.sx

@@ -2605,59 +2605,66 @@
             (let
               ((event-name (parse-compound-event-name)))
               (let
-                ((flt (if (= (tp-type) "bracket-open") (do (adv!) (let ((f (parse-expr))) (if (= (tp-type) "bracket-close") (adv!) nil) f)) nil)))
+                ((of-filter (when (and (= event-name "mutation") (match-kw "of")) (cond ((and (= (tp-type) "ident") (or (= (tp-val) "attributes") (= (tp-val) "childList") (= (tp-val) "characterData"))) (let ((nm (tp-val))) (do (adv!) (dict "type" nm)))) ((= (tp-type) "attr") (let ((attrs (list (tp-val)))) (do (adv!) (define collect-or! (fn () (when (match-kw "or") (cond ((= (tp-type) "attr") (do (set! attrs (append attrs (list (tp-val)))) (adv!) (collect-or!))) (true (set! p (- p 1))))))) (collect-or!) (dict "type" "attrs" "attrs" attrs)))) (true nil)))))
                 (let
-                  ((source (if (match-kw "from") (parse-expr) nil)))
+                  ((flt (if (= (tp-type) "bracket-open") (do (adv!) (let ((f (parse-expr))) (if (= (tp-type) "bracket-close") (adv!) nil) f)) nil)))
                   (let
-                    ((h-margin nil) (h-threshold nil))
+                    ((source (if (match-kw "from") (parse-expr) nil)))
-                    (define
-                      consume-having!
-                      (fn
-                        ()
-                        (cond
-                          ((and (= (tp-type) "ident") (= (tp-val) "having"))
-                            (do
-                              (adv!)
-                              (cond
-                                ((and (= (tp-type) "ident") (= (tp-val) "margin"))
-                                  (do
-                                    (adv!)
-                                    (set! h-margin (parse-expr))
-                                    (consume-having!)))
-                                ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
-                                  (do
-                                    (adv!)
-                                    (set! h-threshold (parse-expr))
-                                    (consume-having!)))
-                                (true nil))))
-                          (true nil))))
-                    (consume-having!)
                     (let
-                      ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
+                      ((h-margin nil) (h-threshold nil))
+                      (define
+                        consume-having!
+                        (fn
+                          ()
+                          (cond
+                            ((and (= (tp-type) "ident") (= (tp-val) "having"))
+                              (do
+                                (adv!)
+                                (cond
+                                  ((and (= (tp-type) "ident") (= (tp-val) "margin"))
+                                    (do
+                                      (adv!)
+                                      (set! h-margin (parse-expr))
+                                      (consume-having!)))
+                                  ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
+                                    (do
+                                      (adv!)
+                                      (set! h-threshold (parse-expr))
+                                      (consume-having!)))
+                                  (true nil))))
+                            (true nil))))
+                      (consume-having!)
                       (let
-                        ((body (parse-cmd-list)))
+                        ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
                         (let
-                          ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                          ((body (parse-cmd-list)))
-                            (finally-clause
-                              (if (match-kw "finally") (parse-cmd-list) nil)))
-                          (match-kw "end")
                           (let
-                            ((parts (list (quote on) event-name)))
+                            ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                              (finally-clause
+                                (if
+                                  (match-kw "finally")
+                                  (parse-cmd-list)
+                                  nil)))
+                            (match-kw "end")
                             (let
-                              ((parts (if every? (append parts (list :every true)) parts)))
+                              ((parts (list (quote on) event-name)))
                               (let
-                                ((parts (if flt (append parts (list :filter flt)) parts)))
+                                ((parts (if every? (append parts (list :every true)) parts)))
                                 (let
-                                  ((parts (if source (append parts (list :from source)) parts)))
+                                  ((parts (if flt (append parts (list :filter flt)) parts)))
                                   (let
-                                    ((parts (if having (append parts (list :having having)) parts)))
+                                    ((parts (if source (append parts (list :from source)) parts)))
                                     (let
-                                      ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                      ((parts (if of-filter (append parts (list :of-filter of-filter)) parts)))
                                       (let
-                                        ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                        ((parts (if having (append parts (list :having having)) parts)))
                                         (let
-                                          ((parts (append parts (list body))))
+                                          ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
-                                          parts))))))))))))))))))
+                                          (let
+                                            ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                            (let
+                                              ((parts (append parts (list body))))
+                                              parts))))))))))))))))))))
       (define
         parse-init-feat
         (fn

shared/static/wasm/sx/hs-runtime.sx

@@ -82,14 +82,36 @@
           observer)))))
 
 ;; Wait for CSS transitions/animations to settle on an element.
-(define hs-init (fn (thunk) (thunk)))
+(define
+  hs-on-mutation-attach!
+  (fn
+    (target mode attr-list)
+    (let
+      ((cfg-attributes (or (= mode "any") (= mode "attributes") (= mode "attrs")))
+        (cfg-childList (or (= mode "any") (= mode "childList")))
+        (cfg-characterData (or (= mode "any") (= mode "characterData"))))
+      (let
+        ((opts (dict "attributes" cfg-attributes "childList" cfg-childList "characterData" cfg-characterData "subtree" true)))
+        (when
+          (and (= mode "attrs") attr-list)
+          (dict-set! opts "attributeFilter" attr-list))
+        (let
+          ((cb (fn (records observer) (dom-dispatch target "mutation" (dict "records" records)))))
+          (let
+            ((observer (host-new "MutationObserver" cb)))
+            (host-call observer "observe" target opts)
+            observer))))))
 
 ;; ── Class manipulation ──────────────────────────────────────────
 
 ;; Toggle a single class on an element.
-(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+(define hs-init (fn (thunk) (thunk)))
 
 ;; Toggle between two classes — exactly one is active at a time.
+(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+
+;; Take a class from siblings — add to target, remove from others.
+;; (hs-take! target cls) — like radio button class behavior
 (begin
   (define
     hs-wait-for
@@ -102,21 +124,20 @@
       (target event-name timeout-ms)
       (perform (list (quote io-wait-event) target event-name timeout-ms)))))
 
-;; Take a class from siblings — add to target, remove from others.
-;; (hs-take! target cls) — like radio button class behavior
-(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-
 ;; ── DOM insertion ───────────────────────────────────────────────
 
 ;; Put content at a position relative to a target.
 ;; pos: "into" | "before" | "after"
-(define
+(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-  hs-toggle-class!
-  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
 
 ;; ── Navigation / traversal ──────────────────────────────────────
 
 ;; Navigate to a URL.
+(define
+  hs-toggle-class!
+  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+
+;; Find next sibling matching a selector (or any sibling).
 (define
   hs-toggle-between!
   (fn
@@ -126,7 +147,7 @@
       (do (dom-remove-class target cls1) (dom-add-class target cls2))
       (do (dom-remove-class target cls2) (dom-add-class target cls1)))))
 
-;; Find next sibling matching a selector (or any sibling).
+;; Find previous sibling matching a selector.
 (define
   hs-toggle-style!
   (fn
@@ -150,7 +171,7 @@
             (dom-set-style target prop "hidden")
             (dom-set-style target prop "")))))))
 
-;; Find previous sibling matching a selector.
+;; First element matching selector within a scope.
 (define
   hs-toggle-style-between!
   (fn
@@ -162,7 +183,7 @@
         (dom-set-style target prop val2)
         (dom-set-style target prop val1)))))
 
-;; First element matching selector within a scope.
+;; Last element matching selector.
 (define
   hs-toggle-style-cycle!
   (fn
@@ -183,7 +204,7 @@
             (true (find-next (rest remaining))))))
       (dom-set-style target prop (find-next vals)))))
 
-;; Last element matching selector.
+;; First/last within a specific scope.
 (define
   hs-take!
   (fn
@@ -223,7 +244,6 @@
               (dom-set-attr target name attr-val)
               (dom-set-attr target name ""))))))))
 
-;; First/last within a specific scope.
 (begin
   (define
     hs-element?
@@ -335,6 +355,9 @@
                 (dom-insert-adjacent-html target "beforeend" value)
                 (hs-boot-subtree! target)))))))))
 
+;; ── Iteration ───────────────────────────────────────────────────
+
+;; Repeat a thunk N times.
 (define
   hs-add-to!
   (fn
@@ -347,9 +370,7 @@
           (append target (list value))))
       (true (do (host-call target "push" value) target)))))
 
-;; ── Iteration ───────────────────────────────────────────────────
+;; Repeat forever (until break — relies on exception/continuation).
-
-;; Repeat a thunk N times.
 (define
   hs-remove-from!
   (fn
@@ -359,7 +380,10 @@
       (filter (fn (x) (not (= x value))) target)
       (host-call target "splice" (host-call target "indexOf" value) 1))))
 
-;; Repeat forever (until break — relies on exception/continuation).
+;; ── Fetch ───────────────────────────────────────────────────────
+
+;; Fetch a URL, parse response according to format.
+;; (hs-fetch url format) — format is "json" | "text" | "html"
 (define
   hs-splice-at!
   (fn
@@ -383,10 +407,10 @@
               (host-call target "splice" i 1))))
         target))))
 
-;; ── Fetch ───────────────────────────────────────────────────────
+;; ── Type coercion ───────────────────────────────────────────────
 
-;; Fetch a URL, parse response according to format.
+;; Coerce a value to a type by name.
-;; (hs-fetch url format) — format is "json" | "text" | "html"
+;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
 (define
   hs-index
   (fn
@@ -398,10 +422,10 @@
       ((string? obj) (nth obj key))
       (true (host-get obj key)))))
 
-;; ── Type coercion ───────────────────────────────────────────────
+;; ── Object creation ─────────────────────────────────────────────
 
-;; Coerce a value to a type by name.
+;; Make a new object of a given type.
-;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
+;; (hs-make type-name) — creates empty object/collection
 (define
   hs-put-at!
   (fn
@@ -423,10 +447,11 @@
                 ((= pos "start") (host-call target "unshift" value)))
               target)))))))
 
-;; ── Object creation ─────────────────────────────────────────────
+;; ── Behavior installation ───────────────────────────────────────
 
-;; Make a new object of a given type.
+;; Install a behavior on an element.
-;; (hs-make type-name) — creates empty object/collection
+;; A behavior is a function that takes (me ...params) and sets up features.
+;; (hs-install behavior-fn me ...args)
 (define
   hs-dict-without
   (fn
@@ -447,27 +472,27 @@
           (host-call (host-global "Reflect") "deleteProperty" out key)
           out)))))
 
-;; ── Behavior installation ───────────────────────────────────────
+;; ── Measurement ─────────────────────────────────────────────────
 
-;; Install a behavior on an element.
+;; Measure an element's bounding rect, store as local variables.
-;; A behavior is a function that takes (me ...params) and sets up features.
+;; Returns a dict with x, y, width, height, top, left, right, bottom.
-;; (hs-install behavior-fn me ...args)
 (define
   hs-set-on!
   (fn
     (props target)
     (for-each (fn (k) (host-set! target k (get props k))) (keys props))))
 
-;; ── Measurement ─────────────────────────────────────────────────
-
-;; Measure an element's bounding rect, store as local variables.
-;; Returns a dict with x, y, width, height, top, left, right, bottom.
-(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
-
 ;; Return the current text selection as a string.  In the browser this is
 ;; `window.getSelection().toString()`.  In the mock test runner, a test
 ;; setup stashes the desired selection text at `window.__test_selection`
 ;; and the fallback path returns that so tests can assert on the result.
+(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
+
+
+;; ── Transition ──────────────────────────────────────────────────
+
+;; Transition a CSS property to a value, optionally with duration.
+;; (hs-transition target prop value duration)
 (define
   hs-ask
   (fn
@@ -476,11 +501,6 @@
       ((w (host-global "window")))
       (if w (host-call w "prompt" msg) nil))))
 
-
-;; ── Transition ──────────────────────────────────────────────────
-
-;; Transition a CSS property to a value, optionally with duration.
-;; (hs-transition target prop value duration)
 (define
   hs-answer
   (fn
@@ -634,6 +654,10 @@
   hs-query-all
   (fn (sel) (host-call (dom-body) "querySelectorAll" sel)))
 
+
+
+
+
 (define
   hs-query-all-in
   (fn
@@ -643,25 +667,21 @@
       (hs-query-all sel)
       (host-call target "querySelectorAll" sel))))
 
-
-
-
-
 (define
   hs-list-set
   (fn
     (lst idx val)
     (append (take lst idx) (cons val (drop lst (+ idx 1))))))
-
-(define
-  hs-to-number
-  (fn (v) (if (number? v) v (or (parse-number (str v)) 0))))
 ;; ── Sandbox/test runtime additions ──────────────────────────────
 ;; Property access — dot notation and .length
+(define
+  hs-to-number
+  (fn (v) (if (number? v) v (or (parse-number (str v)) 0))))
+;; DOM query stub — sandbox returns empty list
 (define
   hs-query-first
   (fn (sel) (host-call (host-global "document") "querySelector" sel)))
-;; DOM query stub — sandbox returns empty list
+;; Method dispatch — obj.method(args)
 (define
   hs-query-last
   (fn
@@ -669,11 +689,11 @@
     (let
       ((all (dom-query-all (dom-body) sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-;; Method dispatch — obj.method(args)
-(define hs-first (fn (scope sel) (dom-query-all scope sel)))
 
 ;; ── 0.9.90 features ─────────────────────────────────────────────
 ;; beep! — debug logging, returns value unchanged
+(define hs-first (fn (scope sel) (dom-query-all scope sel)))
+;; Property-based is — check obj.key truthiness
 (define
   hs-last
   (fn
@@ -681,7 +701,7 @@
     (let
       ((all (dom-query-all scope sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-;; Property-based is — check obj.key truthiness
+;; Array slicing (inclusive both ends)
 (define
   hs-repeat-times
   (fn
@@ -699,7 +719,7 @@
               ((= signal "hs-continue") (do-repeat (+ i 1)))
               (true (do-repeat (+ i 1))))))))
     (do-repeat 0)))
-;; Array slicing (inclusive both ends)
+;; Collection: sorted by
 (define
   hs-repeat-forever
   (fn
@@ -715,7 +735,7 @@
             ((= signal "hs-continue") (do-forever))
             (true (do-forever))))))
     (do-forever)))
-;; Collection: sorted by
+;; Collection: sorted by descending
 (define
   hs-repeat-while
   (fn
@@ -728,7 +748,7 @@
           ((= signal "hs-break") nil)
           ((= signal "hs-continue") (hs-repeat-while cond-fn thunk))
           (true (hs-repeat-while cond-fn thunk)))))))
-;; Collection: sorted by descending
+;; Collection: split by
 (define
   hs-repeat-until
   (fn
@@ -740,7 +760,7 @@
         ((= signal "hs-continue")
           (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))
         (true (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))))))
-;; Collection: split by
+;; Collection: joined by
 (define
   hs-for-each
   (fn
@@ -760,7 +780,7 @@
                 ((= signal "hs-continue") (do-loop (rest remaining)))
                 (true (do-loop (rest remaining))))))))
       (do-loop items))))
-;; Collection: joined by
+
 (begin
   (define
     hs-append

spec/tests/test-hyperscript-behavioral.sx

@@ -8849,9 +8849,22 @@
       (hs-activate! _el-div)
     ))
   (deftest "can listen for attribute mutations"
-    (error "SKIP (skip-list): can listen for attribute mutations"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation of attributes put \"Mutated\" into me")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for attribute mutations on other elements"
-    (error "SKIP (skip-list): can listen for attribute mutations on other elements"))
+    (hs-cleanup!)
+    (let ((_el-d1 (dom-create-element "div")) (_el-d2 (dom-create-element "div")))
+      (dom-set-attr _el-d1 "id" "d1")
+      (dom-set-attr _el-d2 "id" "d2")
+      (dom-set-attr _el-d2 "_" "on mutation of attributes from #d1 put \"Mutated\" into me")
+      (dom-append (dom-body) _el-d1)
+      (dom-append (dom-body) _el-d2)
+      (hs-activate! _el-d2)
+    ))
   (deftest "can listen for characterData mutation filter out other mutations"
     (hs-cleanup!)
     (let ((_el-div (dom-create-element "div")))
@@ -8867,7 +8880,12 @@
       (hs-activate! _el-div)
     ))
   (deftest "can listen for childList mutations"
-    (error "SKIP (skip-list): can listen for childList mutations"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation of childList put \"Mutated\" into me then wait for hyperscript:mutation")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for events in another element (lazy)"
     (hs-cleanup!)
     (let ((_el-div (dom-create-element "div")) (_el-d1 (dom-create-element "div")) (_el-d2 (dom-create-element "div")))
@@ -8880,13 +8898,33 @@
       (hs-activate! _el-div)
     ))
   (deftest "can listen for general mutations"
-    (error "SKIP (skip-list): can listen for general mutations"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation put \"Mutated\" into me then wait for hyperscript:mutation")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for multiple mutations"
-    (error "SKIP (skip-list): can listen for multiple mutations"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation of @foo or @bar put \"Mutated\" into me")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for multiple mutations 2"
-    (error "SKIP (skip-list): can listen for multiple mutations 2"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation of @foo or @bar put \"Mutated\" into me")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for specific attribute mutations"
-    (error "SKIP (skip-list): can listen for specific attribute mutations"))
+    (hs-cleanup!)
+    (let ((_el-div (dom-create-element "div")))
+      (dom-set-attr _el-div "_" "on mutation of @foo put \"Mutated\" into me")
+      (dom-append (dom-body) _el-div)
+      (hs-activate! _el-div)
+    ))
   (deftest "can listen for specific attribute mutations and filter out other attribute mutations"
     (hs-cleanup!)
     (let ((_el-div (dom-create-element "div")))

tests/hs-run-filtered.js

@@ -375,7 +375,115 @@ globalThis.prompt = function(_msg){
 };
 globalThis.Event=Ev; globalThis.CustomEvent=Ev; globalThis.NodeList=Array; globalThis.HTMLCollection=Array;
 globalThis.getComputedStyle=(e)=>e?e.style:{}; globalThis.requestAnimationFrame=(f)=>{f();return 0;};
-globalThis.cancelAnimationFrame=()=>{}; globalThis.MutationObserver=class{observe(){}disconnect(){}};
+globalThis.cancelAnimationFrame=()=>{};
+// HsMutationObserver — cluster-32 mutation mock. Maintains a global
+// registry; setAttribute/appendChild/removeChild/_setInnerHTML hooks below
+// fire matching observers synchronously. A re-entry guard
+// (__hsMutationActive) prevents infinite loops when handler bodies mutate.
+globalThis.__hsMutationRegistry = [];
+globalThis.__hsMutationActive = false;
+function _hsMutAncestorOrEqual(ancestor, target) {
+  let cur = target;
+  while (cur) { if (cur === ancestor) return true; cur = cur.parentElement; }
+  return false;
+}
+function _hsMutMatches(reg, rec) {
+  const o = reg.opts;
+  if (!_hsMutAncestorOrEqual(reg.target, rec.target)) return false;
+  if (rec.type === 'attributes') {
+    if (!o.attributes) return false;
+    if (o.attributeFilter && o.attributeFilter.length > 0) {
+      if (!o.attributeFilter.includes(rec.attributeName)) return false;
+    }
+    return true;
+  }
+  if (rec.type === 'childList') return !!o.childList;
+  if (rec.type === 'characterData') return !!o.characterData;
+  return false;
+}
+function _hsFireMutations(records) {
+  if (globalThis.__hsMutationActive) return;
+  if (!records || records.length === 0) return;
+  const byObs = new Map();
+  for (const r of records) {
+    for (const reg of globalThis.__hsMutationRegistry) {
+      if (!_hsMutMatches(reg, r)) continue;
+      if (!byObs.has(reg.observer)) byObs.set(reg.observer, []);
+      byObs.get(reg.observer).push(r);
+    }
+  }
+  if (byObs.size === 0) return;
+  globalThis.__hsMutationActive = true;
+  try {
+    for (const [obs, recs] of byObs) {
+      try { obs._cb(recs, obs); } catch (e) {}
+    }
+  } finally {
+    globalThis.__hsMutationActive = false;
+  }
+}
+class HsMutationObserver {
+  constructor(cb) { this._cb = cb; this._regs = []; }
+  observe(el, opts) {
+    if (!el) return;
+    // opts is an SX dict: read fields directly. attributeFilter is an SX list
+    // ({_type:'list', items:[...]}) OR a JS array.
+    let af = opts && opts.attributeFilter;
+    if (af && af._type === 'list') af = af.items;
+    const o = {
+      attributes: !!(opts && opts.attributes),
+      childList: !!(opts && opts.childList),
+      characterData: !!(opts && opts.characterData),
+      subtree: !!(opts && opts.subtree),
+      attributeFilter: af || null,
+    };
+    const reg = { observer: this, target: el, opts: o };
+    this._regs.push(reg);
+    globalThis.__hsMutationRegistry.push(reg);
+  }
+  disconnect() {
+    for (const r of this._regs) {
+      const i = globalThis.__hsMutationRegistry.indexOf(r);
+      if (i >= 0) globalThis.__hsMutationRegistry.splice(i, 1);
+    }
+    this._regs = [];
+  }
+  takeRecords() { return []; }
+}
+globalThis.MutationObserver = HsMutationObserver;
+// Hook El prototype methods so mutations fire registered observers.
+// Hooks are no-ops while __hsMutationActive=true (prevents re-entry from
+// handler bodies that themselves mutate the DOM).
+(function _hookElForMutations() {
+  const _setAttr = El.prototype.setAttribute;
+  El.prototype.setAttribute = function(n, v) {
+    const r = _setAttr.call(this, n, v);
+    if (globalThis.__hsMutationRegistry.length)
+      _hsFireMutations([{ type: 'attributes', target: this, attributeName: String(n), oldValue: null }]);
+    return r;
+  };
+  const _append = El.prototype.appendChild;
+  El.prototype.appendChild = function(c) {
+    const r = _append.call(this, c);
+    if (globalThis.__hsMutationRegistry.length)
+      _hsFireMutations([{ type: 'childList', target: this, addedNodes: [c], removedNodes: [] }]);
+    return r;
+  };
+  const _remove = El.prototype.removeChild;
+  El.prototype.removeChild = function(c) {
+    const r = _remove.call(this, c);
+    if (globalThis.__hsMutationRegistry.length)
+      _hsFireMutations([{ type: 'childList', target: this, addedNodes: [], removedNodes: [c] }]);
+    return r;
+  };
+  const _setIH = El.prototype._setInnerHTML;
+  El.prototype._setInnerHTML = function(html) {
+    const r = _setIH.call(this, html);
+    if (globalThis.__hsMutationRegistry.length)
+      _hsFireMutations([{ type: 'childList', target: this, addedNodes: [], removedNodes: [] }]);
+    return r;
+  };
+})();
 // HsResizeObserver — cluster-26 resize mock. Keeps a per-element callback
 // registry so code that observes via `new ResizeObserver(cb)` still works,
 // but HS's `on resize` uses the plain `resize` DOM event dispatched by the
@@ -571,6 +679,8 @@ for(let i=startTest;i<Math.min(endTest,testCount);i++){
   _body.children=[];_body.childNodes=[];_body.innerHTML='';_body.textContent='';
   globalThis.__test_selection='';
   globalThis.__hsCookieStore.clear();
+  globalThis.__hsMutationRegistry.length = 0;
+  globalThis.__hsMutationActive = false;
   globalThis.__currentHsTestName = name;
 
   // Enable step limit for timeout protection

tests/playwright/generate-sx-tests.py

@@ -114,13 +114,6 @@ SKIP_TEST_NAMES = {
     "can filter events based on count range",
     "can filter events based on unbounded count range",
     "can mix ranges",
-    "can listen for general mutations",
-    "can listen for attribute mutations",
-    "can listen for specific attribute mutations",
-    "can listen for childList mutations",
-    "can listen for multiple mutations",
-    "can listen for multiple mutations 2",
-    "can listen for attribute mutations on other elements",
     "each behavior installation has its own event queue",
     "can catch exceptions thrown in js functions",
     "can catch exceptions thrown in hyperscript functions",
@@ -1166,6 +1159,32 @@ def parse_dev_body(body, elements, var_names):
                 ops.append(f'(if (dom-has-class? {target} "{cls}") (dom-remove-class {target} "{cls}") (dom-add-class {target} "{cls}"))')
             continue
 
+        # evaluate(() => document.querySelector(SEL).setAttribute(NAME, VALUE))
+        # — used by mutation tests (cluster 32) to trigger MutationObserver.
+        m = re.match(
+            r'''evaluate\(\s*\(\)\s*=>\s*document\.querySelector\(\s*([\'"])([^\'"]+)\1\s*\)'''
+            r'''\.setAttribute\(\s*([\'"])([\w-]+)\3\s*,\s*([\'"])([^\'"]*)\5\s*\)\s*\)\s*$''',
+            stmt_na, re.DOTALL,
+        )
+        if m and seen_html:
+            sel = re.sub(r'^#work-area\s+', '', m.group(2))
+            target = selector_to_sx(sel, elements, var_names)
+            ops.append(f'(dom-set-attr {target} "{m.group(4)}" "{m.group(6)}")')
+            continue
+
+        # evaluate(() => document.querySelector(SEL).appendChild(document.createElement(TAG)))
+        # — used by mutation childList tests (cluster 32).
+        m = re.match(
+            r'''evaluate\(\s*\(\)\s*=>\s*document\.querySelector\(\s*([\'"])([^\'"]+)\1\s*\)'''
+            r'''\.appendChild\(\s*document\.createElement\(\s*([\'"])([\w-]+)\3\s*\)\s*\)\s*\)\s*$''',
+            stmt_na, re.DOTALL,
+        )
+        if m and seen_html:
+            sel = re.sub(r'^#work-area\s+', '', m.group(2))
+            target = selector_to_sx(sel, elements, var_names)
+            ops.append(f'(dom-append {target} (dom-create-element "{m.group(4)}"))')
+            continue
+
         # evaluate(() => { var range = document.createRange();
         #   var textNode = document.getElementById(ID).firstChild;
         #   range.setStart(textNode, N); range.setEnd(textNode, M);