host: removing a related post no longer clears the relate picker

Bug: the edit page's remove button (on a current relation) was a plain boosted
form — POST /unrelate -> 303 redirect -> the engine re-rendered #content, and the
freshly-swapped relate picker came back EMPTY ("the list of posts to relate" was
cleared).

Fix: make the remove button an AJAX in-place delete, exactly like the relate
candidate rows — each current-relation <li> gets an id and its form carries
sx-post + sx-target=#cur-<kind>-<other> + sx-swap=delete. unrelate-submit returns
an empty 200 for that request so the engine deletes just that one row; #content is
never re-rendered, so the picker is untouched. method+action stay for no-JS.

The empty-200 is gated on the SX-Target header (sent only by the sx-post form), so
a plain boosted form / no-JS POST still redirects + re-renders — the is-a-tag
toggle and graceful degradation are unaffected.

Tests (all red before the fix):
 - lib/host/playwright/relate-picker.spec.js: the remove-button test now asserts
   the picker still has candidates after a removal (the reproduction).
 - web/tests/test-relate-picker.sx: an SX engine test — removing a current relation
   deletes just that row and leaves the sibling picker's list intact.
 - lib/host/tests/blog.sx: the relation-editor renders the AJAX delete attrs;
   unrelate returns empty-200 with SX-Target and 303 without.

Verified: host conformance 275/275, web engine suite 8/8, run-picker-check 2/2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/blog.sx

@@ -434,6 +434,15 @@
 ;; so the blog degrades gracefully to plain server-rendered pages.
 (define host/blog--spa-req? (fn (req) (= (dream-header req "sx-request") "true")))
 
+;; An AJAX in-place row delete vs. a plain boosted form. The engine sends an
+;; SX-Target header for an sx-post form (the relation-editor's remove button,
+;; sx-target=#cur-…), but NOT for a plain boosted form (the is-a-tag toggle). So
+;; this tells "delete just this row, empty 200" apart from "redirect + re-render".
+(define host/blog--row-swap-req?
+  (fn (req)
+    (and (host/blog--spa-req? req)
+         (let ((t (dream-header req "sx-target"))) (and t (not (= t "")))))))
+
 (define host/blog--page
   (fn (req title body)
     (if (host/blog--spa-req? req)
@@ -589,9 +598,18 @@
               (cons (quote ul)
                 (map (fn (s)
                        (quasiquote
-                         (li (a :href (unquote (str "/" s "/")) (unquote s)) " "
+                         ;; AJAX unrelate: sx-post the removal, then sx-swap="delete"
+                         ;; removes just THIS current-relation row (its sx-target) in
+                         ;; place — NO #content re-render, so the relate picker (the
+                         ;; "posts to relate" list) is left untouched. method+action
+                         ;; stay for the no-JS fallback (plain POST -> 303 -> reload).
+                         (li :id (unquote (str "cur-" kind "-" s))
+                           (a :href (unquote (str "/" s "/")) (unquote s)) " "
                            (form :method "post" :style "display:inline"
                              :action (unquote (str "/" slug "/unrelate"))
+                             :sx-post (unquote (str "/" slug "/unrelate"))
+                             :sx-target (unquote (str "#cur-" kind "-" s))
+                             :sx-swap "delete"
                              (input :type "hidden" :name "other" :value (unquote s))
                              (input :type "hidden" :name "kind" :value (unquote kind))
                              (button :type "submit" "remove")))))
@@ -879,7 +897,7 @@
             (dream-redirect (str "/" slug "/edit"))))))))
 
 ;; POST /<slug>/unrelate — remove the relation to `other` under `kind` (default
-;; "related"). Idempotent; redirects back to the edit page.
+;; "related"). Idempotent.
 (define host/blog-unrelate-submit
   (fn (req)
     (let ((slug (dream-param req "slug"))
@@ -888,7 +906,13 @@
       (begin
         (when (and other (not (= other "")) (host/blog--kind-spec kind))
           (host/blog-unrelate! slug other kind))
-        (dream-redirect (str "/" slug "/edit"))))))
+        ;; AJAX in-place remove (the editor's sx-post, carries SX-Target): return an
+        ;; empty 200 so the current-relation row's sx-swap="delete" removes just that
+        ;; one row — no #content re-render, so the relate picker is untouched. A plain
+        ;; boosted form (the tag toggle) or a no-JS POST still redirects + re-renders.
+        (if (host/blog--row-swap-req? req)
+          (dream-html "")
+          (dream-redirect (str "/" slug "/edit")))))))
 
 ;; GET /<slug>/edit — edit form pre-filled with the post's current title, raw
 ;; sx_content (in a textarea — render-to-html escapes the text child, so the

lib/host/playwright/relate-picker.spec.js

@@ -63,10 +63,15 @@ test.describe('relate picker (browser-only)', () => {
     await waitReady(page);
     const relLink = page.locator('a[href="/picker-item-13/"]');
     await expect(relLink).toHaveCount(1);                  // current relation present
-    // click its remove button — a plain boosted form (regression: this did nothing
-    // because bind-boost-form discarded the form's method/action)
+    // the picker is populated (empty filter -> first page of candidates)
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
+    // click its remove button
     await page.locator('li:has(a[href="/picker-item-13/"]) button').click();
     await expect(relLink).toHaveCount(0, { timeout: 12000 });   // relation removed
+    // REGRESSION: removing a relation must NOT clear "the list of posts to relate".
+    // (The old plain-boosted remove form redirected -> re-rendered #content and the
+    // picker came back empty; the AJAX in-place remove leaves the picker untouched.)
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
   });
 
   test('picker populates after a boosted SPA nav to the edit page', async ({ page }) => {

lib/host/tests/blog.sx

@@ -295,6 +295,37 @@
 (host-bl-test "relate-options omits the load-more sentinel on a short last page"
   (contains? (dream-resp-body (host-bl-app (host-bl-req "/alpha-post/relate-options"))) "rp-more")
   false)
+
+;; -- unrelate: AJAX in-place row delete (regression: removing a related post must
+;;    NOT clear the relate picker). The remove button is now an sx-post form that
+;;    deletes just its own current-relation row (sx-target=#cur-…, sx-swap=delete),
+;;    so #content is never re-rendered and the picker is left intact. --
+(host/blog-relate! "alpha-post" "beta-post" "related")
+(host-bl-test "relation-editor remove button is an AJAX in-place delete"
+  (let ((html (render-page (host/blog--relation-editor "alpha-post" "related"))))
+    (list (contains? html "id=\"cur-related-beta-post\"")        ;; row has a target id
+          (contains? html "sx-post=\"/alpha-post/unrelate\"")    ;; AJAX, not plain post
+          (contains? html "sx-target=\"#cur-related-beta-post\"")
+          (contains? html "sx-swap=\"delete\"")))
+  (list true true true true))
+;; the AJAX remove (carries SX-Target) returns an empty 200 so only the row is
+;; swapped out — no redirect, no #content re-render that would blank the picker.
+(host-bl-test "unrelate (AJAX, SX-Target) returns an empty 200"
+  (let ((resp (host/blog-unrelate-submit
+                (dream-request "POST" "/alpha-post/unrelate"
+                  {:sx-request "true" :sx-target "#cur-related-beta-post"}
+                  "other=beta-post&kind=related"))))
+    (list (dream-status resp) (dream-resp-body resp)))
+  (list 200 ""))
+;; a plain boosted form / no-JS POST (no SX-Target) still redirects + re-renders,
+;; so the is-a-tag toggle and graceful degradation are unaffected.
+(host/blog-relate! "alpha-post" "beta-post" "related")
+(host-bl-test "unrelate (plain boosted / no-JS, no SX-Target) still redirects"
+  (dream-status (host/blog-unrelate-submit
+                  (dream-request "POST" "/alpha-post/unrelate"
+                    {:sx-request "true"} "other=beta-post&kind=related")))
+  303)
+(host/blog-unrelate! "alpha-post" "beta-post" "related")
 (host/blog-put! "hint-post" "Hint Post" "(p \"h\")" "published")
 (host-bl-test "relations section: hint when logged-in + no relations"
   (contains? (str (host/blog--relations-or-hint "hint-post" true)) "add some") true)