fix(editor): dismiss the selection popup when its surface leaves the viewport (#425)

The portaled position:fixed add-to-conversation button stays pinned to its
viewport anchor while the sidebar keeps every tab mounted (tab switches only
set display:none; collapsing the panel translates it off-screen), so the
popup could outlive its editor surface. Extract the popup into
useSelectionPopup and close it on any mousedown outside the button, Escape,
document hidden, window blur, and - the tab-switch/collapse path - when an
IntersectionObserver reports the editor surface off-viewport.

src/client/selection-popup.ts

@@ -0,0 +1,157 @@
+/**
+ * The floating "add selection to conversation" popup shared by the text
+ * viewers (markdown preview + the catch-all code viewer): a viewport-anchored
+ * button portaled to `document.body`, kept alive across the selection gesture
+ * and committed on click.
+ *
+ * Dismissal contract (fixes upstream issue #425): the popup must never
+ * outlive its editor surface. The sidebar keeps every tab MOUNTED — switching
+ * tabs only flips the pane cell to `display:none` and collapsing the panel
+ * translates it off-screen — while the portaled `position:fixed` button stays
+ * pinned to its viewport anchor, ignoring both. The caller already hides on
+ * surface scrolls, selection collapse and mode flips; this hook adds the
+ * global dismissal that covers everything else:
+ *
+ * - any `mousedown` outside the button (tab bar, composer, another pane,
+ *   the collapse toggle, …) closes it;
+ * - `Escape` closes it;
+ * - the document going hidden (`visibilitychange`) or the window losing
+ *   focus closes it;
+ * - an `IntersectionObserver` on the editor surface closes it as soon as the
+ *   surface leaves the viewport — the tab-switch (`display:none`) and
+ *   panel-collapse (translated off-screen) paths have no DOM events of their
+ *   own, so the geometry signal is the only reliable one.
+ *
+ * The button's own `mousedown` is never treated as an outside click: the
+ * caller preventDefaults it to keep the selection/caret alive until the
+ * click commits (the hook's capture-phase listener runs first and must not
+ * hide for it).
+ */
+import { useEffect, useRef, useState, type RefObject } from 'react'
+
+/** The floating "add to conversation" action: payload + viewport anchor. */
+export interface SelectionPopup {
+  insert: string
+  left: number
+  top: number
+}
+
+export interface SelectionPopupOptions {
+  /** Commit the payload into the composer draft (button click). */
+  onCommit(insert: string): void
+  /**
+   * The DOM surface that must stay on screen for the popup to live: the
+   * markdown preview container in preview mode, the CodeMirror host
+   * otherwise. Called lazily (refs are null until the content loads).
+   */
+  getSurface(): HTMLElement | null
+}
+
+export interface SelectionPopupControls {
+  /** The current popup (null = hidden). */
+  popup: SelectionPopup | null
+  /** Attach to the portaled button element. */
+  buttonRef: RefObject<HTMLButtonElement>
+  /** Anchor the popup above a selection (viewport-clamped). */
+  show(insert: string, left: number, top: number): void
+  /** Hide the popup (idempotent). */
+  hide(): void
+  /** The button's click: commit the stored payload, then hide. */
+  commit(): void
+}
+
+export function useSelectionPopup(options: SelectionPopupOptions): SelectionPopupControls {
+  // Latest-callback refs: the dismissal listeners live for the mount's
+  // lifetime, so they must not capture stale closures across renders.
+  const onCommitRef = useRef(options.onCommit)
+  const getSurfaceRef = useRef(options.getSurface)
+  onCommitRef.current = options.onCommit
+  getSurfaceRef.current = options.getSurface
+
+  const [popup, setPopup] = useState<SelectionPopup | null>(null)
+  /** Live mirror for click/event-time reads (no re-render race). */
+  const popupRef = useRef<SelectionPopup | null>(null)
+  /** The portaled button itself (for the outside-click guard). */
+  const buttonRef = useRef<HTMLButtonElement>(null)
+  /** The surface visibility observer (created lazily on open). */
+  const observerRef = useRef<IntersectionObserver | null>(null)
+
+  const show = (insert: string, left: number, top: number): void => {
+    const next: SelectionPopup = {
+      insert,
+      left: Math.min(Math.max(left, 80), window.innerWidth - 80),
+      top,
+    }
+    popupRef.current = next
+    setPopup(next)
+  }
+
+  const hide = (): void => {
+    popupRef.current = null
+    setPopup(null)
+  }
+
+  const commit = (): void => {
+    const current = popupRef.current
+    if (current === null) return
+    onCommitRef.current(current.insert)
+    hide()
+  }
+
+  // Global dismissal listeners, registered once; every handler reads the
+  // live popup ref so it no-ops while no popup is open.
+  useEffect(() => {
+    const onMouseDown = (event: MouseEvent): void => {
+      if (popupRef.current === null) return
+      const button = buttonRef.current
+      if (button !== null && (button === event.target || button.contains(event.target as Node))) return
+      hide()
+    }
+    const onKeyDown = (event: KeyboardEvent): void => {
+      if (event.key === 'Escape' && popupRef.current !== null) hide()
+    }
+    const onVisibilityChange = (): void => {
+      if (document.hidden && popupRef.current !== null) hide()
+    }
+    const onWindowBlur = (): void => {
+      if (popupRef.current !== null) hide()
+    }
+    document.addEventListener('mousedown', onMouseDown, true)
+    document.addEventListener('keydown', onKeyDown, true)
+    document.addEventListener('visibilitychange', onVisibilityChange)
+    window.addEventListener('blur', onWindowBlur)
+    return () => {
+      document.removeEventListener('mousedown', onMouseDown, true)
+      document.removeEventListener('keydown', onKeyDown, true)
+      document.removeEventListener('visibilitychange', onVisibilityChange)
+      window.removeEventListener('blur', onWindowBlur)
+      observerRef.current?.disconnect()
+      observerRef.current = null
+    }
+  }, [])
+
+  // Re-target the visibility observer whenever a popup opens: the surface
+  // refs may have been null at mount (content loads async, mode flips swap
+  // the preview container for the CodeMirror host), so the surface is only
+  // trustworthy at open time.
+  useEffect(() => {
+    if (popup === null) return
+    observerRef.current?.disconnect()
+    observerRef.current = null
+    if (typeof IntersectionObserver === 'undefined') return
+    const surface = getSurfaceRef.current()
+    if (surface === null) return
+    const observer = new IntersectionObserver((entries) => {
+      for (const entry of entries) {
+        if (!entry.isIntersecting) hide()
+      }
+    }, { threshold: 0 })
+    observerRef.current = observer
+    observer.observe(surface)
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- the boolean
+    // flip is the only thing that must re-run this; an anchor move while
+    // open keeps the same surface.
+  }, [popup !== null])
+
+  return { popup, buttonRef, show, hide, commit }
+}