HS: when :count changes — scoped watch + parse-cmd feature boundary fix

Three-part fix for element-scoped reactive expressions:

1. Parser: add when/bind to parse-cmd's feature-keyword nil set so
   `... then when X changes ...` is parsed as a new feature, not absorbed
   into the preceding on-handler body as a (ref "when") expression.

2. Parser: parse-when-feat now recognises local (:var) token type so
   `when :count changes ...` dispatches to the when-changes branch.

3. Runtime + compiler: hs-scoped-set! now fires hs-scoped-fire-watchers!
   on change; new hs-scoped-watch! / hs-scoped-fire-watchers! registry;
   compiler emits (hs-scoped-watch! me name (fn (it) body)) for local
   expressions in when-changes AST nodes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/hyperscript/compiler.sx

@@ -2356,14 +2356,26 @@
                 ((= head (quote when-changes))
                   (let
                     ((expr (nth ast 1)) (body (nth ast 2)))
-                    (if
-                      (and (list? expr) (= (first expr) (quote dom-ref)))
-                      (list
-                        (quote hs-dom-watch!)
-                        (hs-to-sx (nth expr 2))
-                        (nth expr 1)
-                        (list (quote fn) (list (quote it)) (hs-to-sx body)))
-                      nil)))
+                    (cond
+                      ((and (list? expr) (= (first expr) (quote dom-ref)))
+                        (list
+                          (quote hs-dom-watch!)
+                          (hs-to-sx (nth expr 2))
+                          (nth expr 1)
+                          (list
+                            (quote fn)
+                            (list (quote it))
+                            (hs-to-sx body))))
+                      ((and (list? expr) (= (first expr) (quote local)))
+                        (list
+                          (quote hs-scoped-watch!)
+                          (quote me)
+                          (nth expr 1)
+                          (list
+                            (quote fn)
+                            (list (quote it))
+                            (hs-to-sx body))))
+                      (true nil))))
                 ((= head (quote init))
                   (list
                     (quote hs-init)

lib/hyperscript/parser.sx

@@ -2840,7 +2840,7 @@
                       ((body (parse-cmd-list)))
                       (match-kw "end")
                       (list (quote view-transition!) using body)))))
-              ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live")))
+              ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live") (= val "when") (= val "bind")))
                 nil)
               (true (parse-expr))))))
       (define
@@ -3060,13 +3060,17 @@
           (define
             plf-skip
             (fn
-              ()
+              (depth)
               (cond
                 ((at-end?) nil)
-                ((and (= (tp-type) "keyword") (or (= (tp-val) "end") (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
                   nil)
-                (true (do (adv!) (plf-skip))))))
-          (plf-skip)
+                ((and (= (tp-type) "keyword") (= (tp-val) "end"))
+                  (if (> depth 0) (do (adv!) (plf-skip (- depth 1))) nil))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "if") (= (tp-val) "repeat")))
+                  (do (adv!) (plf-skip (+ depth 1))))
+                (true (do (adv!) (plf-skip depth))))))
+          (plf-skip 0)
           (match-kw "end")
           (list (quote live-no-op))))
       (define
@@ -3076,15 +3080,20 @@
           (define
             pwf-skip
             (fn
-              ()
+              (depth)
               (cond
                 ((at-end?) nil)
-                ((and (= (tp-type) "keyword") (or (= (tp-val) "end") (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
                   nil)
-                (true (do (adv!) (pwf-skip))))))
+                ((and (= (tp-type) "keyword") (= (tp-val) "end"))
+                  (if (> depth 0) (do (adv!) (pwf-skip (- depth 1))) nil))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "if") (= (tp-val) "repeat")))
+                  (do (adv!) (pwf-skip (+ depth 1))))
+                (true (do (adv!) (pwf-skip depth))))))
           (if
             (or
               (= (tp-type) "hat")
+              (= (tp-type) "local")
               (and (= (tp-type) "keyword") (= (tp-val) "dom")))
             (let
               ((expr (parse-expr)))
@@ -3096,10 +3105,13 @@
                     (match-kw "end")
                     (list (quote when-changes) expr body)))
                 (do
-                  (pwf-skip)
+                  (pwf-skip 0)
                   (match-kw "end")
                   (list (quote when-feat-no-op)))))
-            (do (pwf-skip) (match-kw "end") (list (quote when-feat-no-op))))))
+            (do
+              (pwf-skip 0)
+              (match-kw "end")
+              (list (quote when-feat-no-op))))))
       (define
         parse-bind-feat
         (fn

lib/hyperscript/runtime.sx

@@ -1686,7 +1686,34 @@
 
 (define
   hs-scoped-set!
-  (fn (el name val) (dom-set-data el (str "hs-local-" name) val)))
+  (fn
+    (el name val)
+    (let
+      ((changed (not (= (hs-scoped-get el name) val))))
+      (do
+        (dom-set-data el (str "hs-local-" name) val)
+        (when changed (hs-scoped-fire-watchers! el name val))))))
+
+(begin
+  (define _hs-scoped-watchers (list))
+  (define
+    hs-scoped-watch!
+    (fn
+      (el name handler)
+      (set!
+        _hs-scoped-watchers
+        (cons (list el name handler) _hs-scoped-watchers))))
+  (define
+    hs-scoped-fire-watchers!
+    (fn
+      (el name val)
+      (for-each
+        (fn
+          (entry)
+          (when
+            (and (= (nth entry 0) el) (= (nth entry 1) name))
+            ((nth entry 2) val)))
+        _hs-scoped-watchers))))
 
 (define
   hs-scoped-get
@@ -2701,6 +2728,8 @@
               (if match (dom-parent match) nil)))
           (true el))))))
 
+;; ── SourceInfo API ────────────────────────────────────────────────
+
 (define
   hs-dom-walk
   (fn
@@ -2711,8 +2740,6 @@
       ((= (dom-get-attr el "dom-scope") "isolated") nil)
       (true (hs-dom-walk (dom-parent el) name)))))
 
-;; ── SourceInfo API ────────────────────────────────────────────────
-
 (define
   hs-dom-find-owner
   (fn

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

@@ -2356,14 +2356,26 @@
                 ((= head (quote when-changes))
                   (let
                     ((expr (nth ast 1)) (body (nth ast 2)))
-                    (if
-                      (and (list? expr) (= (first expr) (quote dom-ref)))
-                      (list
-                        (quote hs-dom-watch!)
-                        (hs-to-sx (nth expr 2))
-                        (nth expr 1)
-                        (list (quote fn) (list (quote it)) (hs-to-sx body)))
-                      nil)))
+                    (cond
+                      ((and (list? expr) (= (first expr) (quote dom-ref)))
+                        (list
+                          (quote hs-dom-watch!)
+                          (hs-to-sx (nth expr 2))
+                          (nth expr 1)
+                          (list
+                            (quote fn)
+                            (list (quote it))
+                            (hs-to-sx body))))
+                      ((and (list? expr) (= (first expr) (quote local)))
+                        (list
+                          (quote hs-scoped-watch!)
+                          (quote me)
+                          (nth expr 1)
+                          (list
+                            (quote fn)
+                            (list (quote it))
+                            (hs-to-sx body))))
+                      (true nil))))
                 ((= head (quote init))
                   (list
                     (quote hs-init)

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

@@ -2840,7 +2840,7 @@
                       ((body (parse-cmd-list)))
                       (match-kw "end")
                       (list (quote view-transition!) using body)))))
-              ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live")))
+              ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live") (= val "when") (= val "bind")))
                 nil)
               (true (parse-expr))))))
       (define
@@ -3060,13 +3060,17 @@
           (define
             plf-skip
             (fn
-              ()
+              (depth)
               (cond
                 ((at-end?) nil)
-                ((and (= (tp-type) "keyword") (or (= (tp-val) "end") (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
                   nil)
-                (true (do (adv!) (plf-skip))))))
-          (plf-skip)
+                ((and (= (tp-type) "keyword") (= (tp-val) "end"))
+                  (if (> depth 0) (do (adv!) (plf-skip (- depth 1))) nil))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "if") (= (tp-val) "repeat")))
+                  (do (adv!) (plf-skip (+ depth 1))))
+                (true (do (adv!) (plf-skip depth))))))
+          (plf-skip 0)
           (match-kw "end")
           (list (quote live-no-op))))
       (define
@@ -3076,15 +3080,20 @@
           (define
             pwf-skip
             (fn
-              ()
+              (depth)
               (cond
                 ((at-end?) nil)
-                ((and (= (tp-type) "keyword") (or (= (tp-val) "end") (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "on") (= (tp-val) "init") (= (tp-val) "def") (= (tp-val) "behavior") (= (tp-val) "live") (= (tp-val) "when")))
                   nil)
-                (true (do (adv!) (pwf-skip))))))
+                ((and (= (tp-type) "keyword") (= (tp-val) "end"))
+                  (if (> depth 0) (do (adv!) (pwf-skip (- depth 1))) nil))
+                ((and (= (tp-type) "keyword") (or (= (tp-val) "if") (= (tp-val) "repeat")))
+                  (do (adv!) (pwf-skip (+ depth 1))))
+                (true (do (adv!) (pwf-skip depth))))))
           (if
             (or
               (= (tp-type) "hat")
+              (= (tp-type) "local")
               (and (= (tp-type) "keyword") (= (tp-val) "dom")))
             (let
               ((expr (parse-expr)))
@@ -3096,10 +3105,13 @@
                     (match-kw "end")
                     (list (quote when-changes) expr body)))
                 (do
-                  (pwf-skip)
+                  (pwf-skip 0)
                   (match-kw "end")
                   (list (quote when-feat-no-op)))))
-            (do (pwf-skip) (match-kw "end") (list (quote when-feat-no-op))))))
+            (do
+              (pwf-skip 0)
+              (match-kw "end")
+              (list (quote when-feat-no-op))))))
       (define
         parse-bind-feat
         (fn

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

@@ -1686,7 +1686,34 @@
 
 (define
   hs-scoped-set!
-  (fn (el name val) (dom-set-data el (str "hs-local-" name) val)))
+  (fn
+    (el name val)
+    (let
+      ((changed (not (= (hs-scoped-get el name) val))))
+      (do
+        (dom-set-data el (str "hs-local-" name) val)
+        (when changed (hs-scoped-fire-watchers! el name val))))))
+
+(begin
+  (define _hs-scoped-watchers (list))
+  (define
+    hs-scoped-watch!
+    (fn
+      (el name handler)
+      (set!
+        _hs-scoped-watchers
+        (cons (list el name handler) _hs-scoped-watchers))))
+  (define
+    hs-scoped-fire-watchers!
+    (fn
+      (el name val)
+      (for-each
+        (fn
+          (entry)
+          (when
+            (and (= (nth entry 0) el) (= (nth entry 1) name))
+            ((nth entry 2) val)))
+        _hs-scoped-watchers))))
 
 (define
   hs-scoped-get
@@ -2701,6 +2728,8 @@
               (if match (dom-parent match) nil)))
           (true el))))))
 
+;; ── SourceInfo API ────────────────────────────────────────────────
+
 (define
   hs-dom-walk
   (fn
@@ -2711,8 +2740,6 @@
       ((= (dom-get-attr el "dom-scope") "isolated") nil)
       (true (hs-dom-walk (dom-parent el) name)))))
 
-;; ── SourceInfo API ────────────────────────────────────────────────
-
 (define
   hs-dom-find-owner
   (fn