📌 fix: Pin Scroll-to-Bottom Rib in Message Nav (#14397)

* 📌 fix: Pin Scroll-to-Bottom Rib in Message Nav

Render the terminus rib outside the scrolling rail, between the column
and the down chevron, so the scroll-to-bottom affordance stays in view
no matter how far the rail has scrolled.

- scrubTo enumerates ribs from the nav so drag-to-bottom still lands on
  the terminus
- the pinned rib drives the shared preview itself on hover and focus,
  since it is no longer covered by the column's pointer magnification

* 🖱️ fix: Keep Drag-Scrub Startable From the Pinned Terminus

Pointer-down on the pinned rib no longer reaches the column's handler now
that it renders outside the scrollport, so wire the same drag-start to the
wrapper. Dragging up from the bottom dot scrubs the thread again.

client/src/components/Chat/Messages/MessageNav.tsx

@@ -313,6 +313,16 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
     return map;
   }, [entries]);
 
+  /** The terminus rib is pinned beside the down chevron rather than living in
+   *  the scrolling column, so it stays reachable however far the rail scrolls. */
+  const { messageEntries, endEntry } = useMemo(() => {
+    const last = entries[entries.length - 1];
+    if (last?.isEnd === true) {
+      return { messageEntries: entries.slice(0, -1), endEntry: last };
+    }
+    return { messageEntries: entries, endEntry: null };
+  }, [entries]);
+
   const getCurrentVisibleId = useCallback((): string | null => {
     const container = scrollableRef.current;
     if (!container) {
@@ -518,10 +528,11 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
   const scrubTo = useCallback(
     (clientY: number) => {
       const col = columnRef.current;
-      if (!col) {
+      const nav = navRef.current;
+      if (!col || !nav) {
         return;
       }
-      const ribs = col.querySelectorAll<HTMLElement>('[data-msg-id]');
+      const ribs = nav.querySelectorAll<HTMLElement>('[data-msg-id]');
       const count = ribs.length;
       if (count === 0) {
         return;
@@ -715,6 +726,38 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
     [positionTip, revealTip],
   );
 
+  /** The pinned terminus lives outside the column, so it drives the shared
+   *  preview itself instead of through the rail's pointer magnification. */
+  const showEndTip = useCallback(
+    (el: HTMLElement) => {
+      const rect = el.getBoundingClientRect();
+      const left = columnRef.current?.getBoundingClientRect().left ?? rect.left;
+      focusTooltip(MESSAGES_END_ID, rect.top + rect.height / 2, window.innerWidth - left + 8);
+    },
+    [focusTooltip],
+  );
+
+  const handleEndPointerEnter = useCallback(
+    (e: React.PointerEvent<HTMLDivElement>) => showEndTip(e.currentTarget),
+    [showEndTip],
+  );
+
+  const handleEndFocus = useCallback(
+    (e: React.FocusEvent<HTMLDivElement>) => showEndTip(e.currentTarget),
+    [showEndTip],
+  );
+
+  const handleEndBlur = useCallback(
+    (e: React.FocusEvent<HTMLDivElement>) => {
+      const next = e.relatedTarget as Node | null;
+      if (next && e.currentTarget.contains(next)) {
+        return;
+      }
+      clearTooltip();
+    },
+    [clearTooltip],
+  );
+
   const applyMagnify = useCallback(() => {
     magRafRef.current = null;
     const col = columnRef.current;
@@ -1209,9 +1252,7 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
     return () => document.removeEventListener('keydown', onKeyDown);
   }, [focusNav]);
 
-  const hasEnd = entries.length > 0 && entries[entries.length - 1].isEnd === true;
-  const messageCount = hasEnd ? entries.length - 1 : entries.length;
-  if (messageCount < 3) {
+  if (messageEntries.length < 3) {
     return null;
   }
 
@@ -1252,15 +1293,11 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
         className="flex min-h-0 w-14 cursor-pointer touch-none select-none flex-col items-stretch gap-1.5 overflow-y-auto [&::-webkit-scrollbar]:hidden"
         style={{ scrollbarWidth: 'none', msOverflowStyle: 'none' }}
       >
-        {entries.map((entry) => {
-          const label = entry.isEnd
-            ? localize('com_ui_scroll_to_bottom')
-            : localize(
-                entry.isUser
-                  ? 'com_ui_message_nav_go_to_user'
-                  : 'com_ui_message_nav_go_to_assistant',
-                { 0: entry.preview.slice(0, 30) },
-              );
+        {messageEntries.map((entry) => {
+          const label = localize(
+            entry.isUser ? 'com_ui_message_nav_go_to_user' : 'com_ui_message_nav_go_to_assistant',
+            { 0: entry.preview.slice(0, 30) },
+          );
           const isHighlighted =
             hoveredId != null ? hoveredId === entry.id : visibleIds.has(entry.id);
           return (
@@ -1276,6 +1313,27 @@ function MessageNav({ scrollableRef }: { scrollableRef: React.RefObject<HTMLDivE
         })}
       </div>
 
+      {endEntry && (
+        <div
+          className="flex w-14 cursor-pointer touch-none select-none flex-col items-stretch"
+          onPointerDown={handlePointerDown}
+          onPointerEnter={handleEndPointerEnter}
+          onPointerLeave={clearTooltip}
+          onFocus={handleEndFocus}
+          onBlur={handleEndBlur}
+        >
+          <MessageIndicator
+            entry={endEntry}
+            isHighlighted={
+              hoveredId != null ? hoveredId === endEntry.id : visibleIds.has(endEntry.id)
+            }
+            isCurrent={currentId === endEntry.id}
+            onSelect={handleSelect}
+            label={localize('com_ui_scroll_to_bottom')}
+          />
+        </div>
+      )}
+
       <button
         type="button"
         onClick={jumpToNext}

client/src/components/Chat/Messages/__tests__/MessageNav.spec.tsx

@@ -1590,6 +1590,121 @@ describe('MessageNav', () => {
       expect(ribs[3].getAttribute('aria-label')).toBe('com_ui_scroll_to_bottom');
     });
 
+    it('pins the terminus outside the scrolling column, between it and the next chevron', () => {
+      const { container } = renderNavWithEnd(threeMessages());
+      const nav = container.querySelector('nav') as HTMLElement;
+      const column = container.querySelector('nav > div') as HTMLDivElement;
+
+      expect(column.querySelector('[data-msg-id="messages-end"]')).toBeNull();
+      expect(nav.querySelector('[data-msg-id="messages-end"]')).not.toBeNull();
+
+      const kids = Array.from(nav.children);
+      const endIndex = kids.findIndex((k) => k.querySelector('[data-msg-id="messages-end"]'));
+      const nextIndex = kids.findIndex(
+        (k) => k.getAttribute('aria-label') === 'com_ui_message_nav_next',
+      );
+      expect(endIndex).toBe(kids.indexOf(column) + 1);
+      expect(nextIndex).toBe(endIndex + 1);
+    });
+
+    it('keeps column children aligned with message entries once the terminus is pinned', () => {
+      const messages = Array.from({ length: 5 }, (_, i) =>
+        buildMessage({ messageId: `m-${i}`, text: `message ${i}`, isCreatedByUser: i % 2 === 0 }),
+      );
+      const { container } = renderNavWithEnd(messages);
+      const column = container.querySelector('nav > div') as HTMLDivElement;
+
+      expect(column.children).toHaveLength(messages.length);
+      for (let i = 0; i < messages.length; i++) {
+        expect(column.children[i].getAttribute('data-msg-id')).toBe(`m-${i}`);
+      }
+    });
+
+    it('centers the column on the visible window using the pinned-out child indices', () => {
+      const messages = Array.from({ length: 10 }, (_, i) =>
+        buildMessage({ messageId: `m-${i}`, text: `message ${i}`, isCreatedByUser: i % 2 === 0 }),
+      );
+      const { container, scrollable } = renderNavWithEnd(messages);
+      const column = container.querySelector('nav > div') as HTMLDivElement;
+
+      Object.defineProperty(column, 'clientHeight', { value: 30, configurable: true });
+      Object.defineProperty(column, 'scrollHeight', { value: 200, configurable: true });
+      Object.defineProperty(column, 'scrollTop', { value: 0, writable: true, configurable: true });
+      for (let i = 0; i < column.children.length; i++) {
+        Object.defineProperty(column.children[i], 'offsetTop', { value: i * 10 });
+        Object.defineProperty(column.children[i], 'offsetHeight', { value: 6 });
+      }
+      (scrollable as HTMLElement).scrollTop = 400;
+
+      act(() => {
+        fireEvent.scroll(scrollable);
+        jest.advanceTimersByTime(32);
+      });
+
+      /** Visible rows m-1..m-4 → mid of ribs 1 and 4, minus half the column height. */
+      expect(column.scrollTop).toBe(13);
+    });
+
+    it('starts a scrub drag from the pinned terminus', () => {
+      const messages = Array.from({ length: 5 }, (_, i) =>
+        buildMessage({ messageId: `m-${i}`, text: `message ${i}`, isCreatedByUser: i % 2 === 0 }),
+      );
+      const { container, scrollable } = renderNavWithEnd(messages);
+      const column = container.querySelector('nav > div') as HTMLDivElement;
+      column.getBoundingClientRect = () => ({ top: 0, bottom: 50, height: 50 }) as DOMRect;
+      const wrapper = container.querySelector('[data-msg-id="messages-end"]')!
+        .parentElement as HTMLElement;
+      const getById = jest.spyOn(document, 'getElementById');
+
+      act(() => {
+        fireEvent.pointerDown(wrapper, { pointerId: 1, button: 0, buttons: 1, clientY: 60 });
+        fireEvent.pointerMove(document, { pointerId: 1, buttons: 1, clientY: 0 });
+      });
+
+      expect(getById.mock.calls.map((c) => c[0])).toContain('m-0');
+      getById.mockRestore();
+      expect(scrollable).toBeDefined();
+    });
+
+    it('previews the terminus on hover even though it sits outside the column', () => {
+      const { container } = renderNavWithEnd(threeMessages());
+      const wrapper = container.querySelector('[data-msg-id="messages-end"]')!
+        .parentElement as HTMLElement;
+
+      act(() => {
+        fireEvent.pointerEnter(wrapper, { pointerId: 1, clientY: 5 });
+        jest.advanceTimersByTime(80);
+      });
+      expect(document.body.querySelector('[role="tooltip"]')).toHaveTextContent(
+        'com_ui_scroll_to_bottom',
+      );
+
+      act(() => {
+        fireEvent.pointerLeave(wrapper, { pointerId: 1 });
+      });
+      expect(document.body.querySelector('[role="tooltip"]')).toBeNull();
+    });
+
+    it('previews the terminus when it takes keyboard focus', () => {
+      const { container } = renderNavWithEnd(threeMessages());
+      const endRib = container.querySelector('[data-msg-id="messages-end"]') as HTMLElement;
+
+      act(() => {
+        endRib.dispatchEvent(new FocusEvent('focusin', { bubbles: true }));
+        jest.advanceTimersByTime(80);
+      });
+      expect(document.body.querySelector('[role="tooltip"]')).toHaveTextContent(
+        'com_ui_scroll_to_bottom',
+      );
+
+      act(() => {
+        endRib.dispatchEvent(
+          new FocusEvent('focusout', { bubbles: true, relatedTarget: document.body }),
+        );
+      });
+      expect(document.body.querySelector('[role="tooltip"]')).toBeNull();
+    });
+
     it('omits the terminus indicator and the nav when there are fewer than 3 messages', () => {
       const messages = [
         buildMessage({ messageId: 'a', text: 'alpha', isCreatedByUser: true }),