diff --git a/apps/docs/editor/custom-ui/api-reference.mdx b/apps/docs/editor/custom-ui/api-reference.mdx
index 07fbebb188..2033c484f7 100644
--- a/apps/docs/editor/custom-ui/api-reference.mdx
+++ b/apps/docs/editor/custom-ui/api-reference.mdx
@@ -175,6 +175,8 @@ ui.comments.reply(parentCommentId, { text });
 ui.comments.resolve(commentId);
 ui.comments.reopen(commentId);
 ui.comments.delete(commentId);
+ui.comments.setActive(commentId); // highlight without scrolling or moving selection
+ui.comments.setActive(null); // clear active highlight
 ui.comments.scrollTo(commentId);
 ```
 
@@ -242,8 +244,10 @@ Geometry. Browser-only.
 ```ts
 ui.viewport.getRect({ target: { kind: 'entity', entityType: 'comment', entityId } });
 await ui.viewport.scrollIntoView({ target, block: 'center', behavior: 'smooth' });
+ui.viewport.observe(({ reason }) => {});
 
 ui.viewport.getHost(); // painted host element | null
+ui.viewport.getScrollContainer(); // scroll element | null
 ui.viewport.entityAt({ x, y }); // ViewportEntityHit[]
 ui.viewport.positionAt({ x, y }); // ViewportPositionHit | null
 ui.viewport.contextAt({ x, y }); // ViewportContext (always returns)
diff --git a/apps/docs/editor/custom-ui/comments.mdx b/apps/docs/editor/custom-ui/comments.mdx
index 54ad352cc1..211e8dd68c 100644
--- a/apps/docs/editor/custom-ui/comments.mdx
+++ b/apps/docs/editor/custom-ui/comments.mdx
@@ -126,6 +126,36 @@ export function CommentComposer({ onPosted }: { onPosted(): void }) {
 
 See the [selection capture example](https://github.com/superdoc-dev/superdoc/tree/main/examples/editor/custom-ui/selection-capture) for a runnable vanilla version: open a composer, move focus to a textarea, and post against the original selection.
 
+### When the built-in bubble opens your composer
+
+If you keep the built-in floating comment bubble but render your own composer, listen for the pending comment event. The event carries `pendingSelection`, captured before SuperDoc inserts the pending mark, so you do not need to continuously cache the last non-empty selection.
+
+```tsx
+import { useEffect, useState } from 'react';
+import type { SuperDocCommentsUpdatePayload } from 'superdoc';
+import type { SelectionInfo } from 'superdoc/ui';
+
+const [pendingSelection, setPendingSelection] = useState<SelectionInfo | null>(null);
+
+useEffect(() => {
+  if (!superdoc) return;
+
+  const handleCommentsUpdate = ({ type, pendingSelection }: SuperDocCommentsUpdatePayload) => {
+    if (type === 'pending') setPendingSelection(pendingSelection ?? null);
+  };
+
+  superdoc.on('comments-update', handleCommentsUpdate);
+  return () => superdoc.off('comments-update', handleCommentsUpdate);
+}, [superdoc]);
+
+function postComment(text: string) {
+  if (!ui || !pendingSelection) return;
+  return ui.comments.createFromCapture(pendingSelection, { text: text.trim() });
+}
+```
+
+`pendingSelection` is a Document API selection snapshot. It has the `target` that `createFromCapture` needs, but it can be `null` for PDF selections, mismatched documents, or selections with no text anchor.
+
 ## Resolve and reopen
 
 ```tsx
@@ -136,6 +166,17 @@ ui.comments.delete(commentId);
 
 All three return a Document API receipt. The next snapshot from `useSuperDocComments()` reflects the change.
 
+## Activate a comment highlight without scrolling
+
+Use `setActive` when your UI handles scrolling and only needs SuperDoc to highlight the comment.
+
+```tsx
+scrollYourSidebarOrEditor(commentId);
+const activated = ui.comments.setActive(commentId);
+```
+
+Pass `null` to clear the highlight. Reply ids activate their anchored thread root, unknown ids return `false`, and a later editor selection change can clear the highlight because `setActive` does not move the caret.
+
 ## Scroll to a comment
 
 ```tsx
diff --git a/apps/docs/editor/custom-ui/selection-and-viewport.mdx b/apps/docs/editor/custom-ui/selection-and-viewport.mdx
index df4260521c..b0db6bfd5e 100644
--- a/apps/docs/editor/custom-ui/selection-and-viewport.mdx
+++ b/apps/docs/editor/custom-ui/selection-and-viewport.mdx
@@ -116,12 +116,7 @@ export function SelectionPopover() {
     }
     const update = () => setRect(ui.selection.getAnchorRect({ placement: 'start' }));
     update();
-    window.addEventListener('scroll', update, true);
-    window.addEventListener('resize', update);
-    return () => {
-      window.removeEventListener('scroll', update, true);
-      window.removeEventListener('resize', update);
-    };
+    return ui.viewport.observe(update);
   }, [ui, selection.empty, selection.target, selection.quotedText]);
 
   if (!rect) return null;
@@ -182,6 +177,17 @@ document.addEventListener('contextmenu', (event) => {
 
 The painted host is separate from the offscreen ProseMirror DOM. Reach for `getHost()` when you need the element a `MouseEvent.target` would resolve to inside the editor.
 
+## Get the scroll container
+
+Use `ui.viewport.getScrollContainer()` when your UI needs to scroll the editor or attach scroll listeners. It returns `null` when the page window is the scroller.
+
+```ts
+const scroller = ui.viewport.getScrollContainer();
+(scroller ?? window).scrollTo({ top: 0, behavior: 'smooth' });
+```
+
+If you cache rects from `getRect`, subscribe to `ui.viewport.observe(() => remeasure())` and re-query them when geometry changes.
+
 ## The right-click bundle
 
 `ui.viewport.contextAt({ x, y })` composes the four primitives above into one bundle. Use it for custom right-click menus where the same shape filters items and runs handlers. See [Custom right-click menu](/editor/custom-ui/context-menu) for the full pattern.
@@ -231,6 +237,14 @@ if (result.success) {
 }
 ```
 
+For a body text range, pass the same `TextAddress` or `TextTarget` shape you get from selection and Document API calls:
+
+```ts
+const result = ui.viewport.getRect({
+  target: { kind: 'text', blockId: 'p-123', range: { start: 8, end: 21 } },
+});
+```
+
 Rects are plain values in viewport coordinates. Multi-page or multi-line targets return `rects` carrying every painted occurrence in document order.
 
 `success: false` reasons:
@@ -245,6 +259,6 @@ Rects are plain values in viewport coordinates. Multi-page or multi-line targets
 ## Trade-offs
 
 - The selection slice updates on every editor change. Subscribe via the typed hook, not via raw `ui.select(...)`, so the controller can dedupe by shallow equality.
-- `getRect`, `getRects`, `getAnchorRect`, `entityAt`, and `positionAt` are synchronous and DOM-driven. Run them inside a `useLayoutEffect` if you need them to settle before paint. Text-anchored `getRect` targets are deferred until story-aware text resolution lands.
+- `getRect`, `getRects`, `getAnchorRect`, `entityAt`, and `positionAt` are synchronous and DOM-driven. Run them inside a `useLayoutEffect` if you need them to settle before paint. Body text-range `getRect` targets are supported; header, footer, footnote, and endnote text targets are deferred until story-aware text resolution lands.
 - `scrollIntoView` honors `behavior: 'smooth'` for body content. Non-body entities (header / footer / footnote / endnote) snap to view because story activation has to mount the surface synchronously before alignment. If smooth animation across non-body entities matters, scroll the editor surface yourself first with `behavior: 'smooth'`, then call `scrollIntoView` to land the entity.
 - `contextAt({ x, y })` always returns a bundle. Non-numeric coords coerce to `(0, 0)`, which is a real point and may sit inside the painted host. Pass real coordinates if you want the result to reflect a specific click.
diff --git a/demos/editor/custom-ui/README.md b/demos/editor/custom-ui/README.md
index e0fcaff3b8..879c931923 100644
--- a/demos/editor/custom-ui/README.md
+++ b/demos/editor/custom-ui/README.md
@@ -44,7 +44,7 @@ The demo composes anchored citation pointers on top of `editor.doc.metadata.*` a
 | **Mock RAG output** | Pre-canned text + per-citation payloads (sourceId, displayText, locator, excerpt, confidence). Stand-in for a real generation pipeline. | `mockDraft.ts` |
 | **Insert + attach** | Inserts the text via `editor.doc.insert`, then computes a `SelectionTarget` for each cited span and calls `editor.doc.metadata.attach` per citation. | `GenerateDraftButton.tsx`, `useCitations.ts` |
 | **Sources panel** | Lists citations grouped by `sourceId`. Scroll-to navigation uses `ui.metadata.scrollIntoView({ id })`. Edit form calls `editor.doc.metadata.update`. | `CitationsPanel.tsx` |
-| **Highlight overlay** | Renders one absolute-positioned rectangle per painted line of each cited span. Rects come from `ui.metadata.getRect({ id })`. Remeasures on scroll, resize, ResizeObserver, and MutationObserver. | `CitationHighlights.tsx` |
+| **Highlight overlay** | Renders one absolute-positioned rectangle per painted line of each cited span. Rects come from `ui.metadata.getRect({ id })` and remeasure through `ui.viewport.observe`. | `CitationHighlights.tsx` |
 | **Hover popover** | `ui.viewport.entityAt({ x, y })` returns the content control under the cursor; `metadata.get({ id })` fetches the payload to render. | `CitationPopover.tsx` |
 | **Persistence** | Hidden inline content controls in the body carry the stable id in `w:tag`; payloads live in a namespaced custom XML data part. Survives DOCX export, reopen, and Word save (validated by the `word-roundtrip` fixtures in the monorepo). | `editor.doc.metadata.*` |
 
diff --git a/demos/editor/custom-ui/src/components/CitationHighlights.tsx b/demos/editor/custom-ui/src/components/CitationHighlights.tsx
index cb48330f14..bfb2eb19bd 100644
--- a/demos/editor/custom-ui/src/components/CitationHighlights.tsx
+++ b/demos/editor/custom-ui/src/components/CitationHighlights.tsx
@@ -5,26 +5,7 @@ import { useCitations } from './useCitations';
 
 /**
  * Renders absolute-positioned overlay rectangles on every cited span.
- *
- * Geometry comes from `ui.metadata.getRect({ id })`. The handle hides
- * the underlying lookup (metadata id = SDT `w:tag` → SDT node id →
- * painter rect) so consumers only carry the metadata id they originally
- * passed to `editor.doc.metadata.attach`. Before SD-3204 this demo had
- * to compose `useSuperDocContentControls` + a tag → nodeId map +
- * `ui.contentControls.getRect`; that bridge is now obviated.
- *
- * `getRect` returns `rects[]` on success (one ViewportRect per painted
- * line of a wrapped span), so line-wrapped citations get clean per-line
- * underlines without spilling across the page margin.
- *
- * Remeasure triggers: window scroll/resize, ResizeObserver on the
- * editor canvas (catches pagination/zoom), and MutationObserver on
- * the canvas DOM (catches text edits that move cited spans without
- * resizing the canvas). All paths funnel through a single rAF tick
- * to coalesce bursts of keystrokes into one remeasure per frame.
- *
- * This is demo-tier instrumentation. A library would expose a
- * dedicated layout/transaction event rather than observing the DOM.
+ * `ui.viewport.observe` tells the demo when cached rects may have moved.
  */
 type HighlightEntry = { metadataId: string; tooltip: string; rects: ViewportRect[] };
 
@@ -35,7 +16,8 @@ export function CitationHighlights() {
 
   useEffect(() => {
     const metadata = ui?.metadata;
-    if (!metadata?.getRect) {
+    const viewport = ui?.viewport;
+    if (!metadata?.getRect || !viewport?.observe) {
       setEntries([]);
       return;
     }
@@ -54,8 +36,7 @@ export function CitationHighlights() {
       setEntries(next);
     };
 
-    // Coalesce burst triggers (multi-mutation keystrokes, ResizeObserver
-    // firing alongside MutationObserver, etc.) into one remeasure per frame.
+    // Coalesce burst geometry events into one remeasure per frame.
     let rafHandle: number | null = null;
     const scheduleRemeasure = () => {
       if (rafHandle !== null) return;
@@ -66,28 +47,10 @@ export function CitationHighlights() {
     };
 
     remeasure();
-    window.addEventListener('scroll', scheduleRemeasure, true);
-    window.addEventListener('resize', scheduleRemeasure);
-
-    const canvas = document.querySelector('.editor-canvas');
-    const resizeObserver = canvas ? new ResizeObserver(scheduleRemeasure) : null;
-    if (canvas && resizeObserver) resizeObserver.observe(canvas);
-
-    // Skip the DOM-mutation observer when there are no citations to track,
-    // so the demo doesn't observe the editor body when there's nothing to update.
-    const mutationObserver =
-      canvas && citations.length > 0
-        ? new MutationObserver(scheduleRemeasure)
-        : null;
-    if (canvas && mutationObserver) {
-      mutationObserver.observe(canvas, { childList: true, subtree: true, characterData: true });
-    }
+    const stopObservingViewport = viewport.observe(scheduleRemeasure);
 
     return () => {
-      window.removeEventListener('scroll', scheduleRemeasure, true);
-      window.removeEventListener('resize', scheduleRemeasure);
-      resizeObserver?.disconnect();
-      mutationObserver?.disconnect();
+      stopObservingViewport();
       if (rafHandle !== null) cancelAnimationFrame(rafHandle);
     };
   }, [ui, citations]);
diff --git a/demos/editor/custom-ui/src/components/SelectionPopover.tsx b/demos/editor/custom-ui/src/components/SelectionPopover.tsx
index 84c1f7e91b..723e8a83e2 100644
--- a/demos/editor/custom-ui/src/components/SelectionPopover.tsx
+++ b/demos/editor/custom-ui/src/components/SelectionPopover.tsx
@@ -18,10 +18,9 @@ const ANCHOR_GAP = 8;
  * the wrong place. `ui.selection.getAnchorRect()` reads from the
  * painted layout instead.
  *
- * The popover only re-positions when the selection slice meaningfully
- * changes (range / quoted text). Scroll and resize trigger a refresh
- * so the anchor stays glued through layout shifts; the rect is
- * viewport-relative so `position: fixed` is enough.
+ * The popover re-positions when the selection changes or viewport
+ * geometry invalidates. Rects are viewport-relative, so
+ * `position: fixed` is enough.
  */
 export function SelectionPopover({ onComposeComment }: Props) {
   const ui = useSuperDocUI();
@@ -39,16 +38,7 @@ export function SelectionPopover({ onComposeComment }: Props) {
       setRect(ui.selection.getAnchorRect({ placement: 'start' }));
     };
     update();
-    // Scroll-capture listener so the popover follows the page when the
-    // user scrolls. The document is paginated so scroll happens
-    // somewhere up the DOM chain; `capture: true` catches scroll on
-    // any scrollable ancestor.
-    window.addEventListener('scroll', update, true);
-    window.addEventListener('resize', update);
-    return () => {
-      window.removeEventListener('scroll', update, true);
-      window.removeEventListener('resize', update);
-    };
+    return ui.viewport.observe(update);
   }, [ui, selection.empty, selection.target, selection.quotedText]);
 
   // Clamp horizontally inside the viewport and flip below the selection