fix(chat): restore the 40px end-follow threshold

The half-viewport band made return-to-end detection too broad. Restore the fixed 40px threshold without changing gesture handling or follow transitions.

Testing: 14 focused scroll tests pass; UI type-check passes; UI lint has one unrelated existing warning; dead-code report inspected. Live chat behavior not validated.

packages/ui/src/components/chat/lib/scroll/timelineScrollAnchoring.test.ts

@@ -94,11 +94,11 @@ describe('resolveRealContentEndOffset', () => {
 });
 
 describe('resolveTimelineIsAtEnd', () => {
-    test('counts half a viewport from the full content length as the end', () => {
+    test('uses a 40px band regardless of viewport height', () => {
         expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1400, scrollLength: 600 })).toBe(true);
-        expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1100, scrollLength: 600 })).toBe(true);
+        expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1360, scrollLength: 600 })).toBe(true);
-        expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1099, scrollLength: 600 })).toBe(false);
+        expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1359, scrollLength: 600 })).toBe(false);
-        // Tiny viewports keep a 40px floor.
+        expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1100, scrollLength: 600 })).toBe(false);
         expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1900, scrollLength: 60 })).toBe(true);
         expect(resolveTimelineIsAtEnd({ contentLength: 2000, scroll: 1899, scrollLength: 60 })).toBe(false);
     });

packages/ui/src/components/chat/lib/scroll/timelineScrollAnchoring.ts

@@ -61,17 +61,8 @@ export const resolveRealContentEndOffset = ({
     return Math.max(0, lastBottom + Math.max(0, footerSize) - visibleLength);
 };
 
-// "At the end" for follow purposes is half a viewport. Leaving the end is
+// Keep return-to-end detection in a tight band, rather than half a viewport.
-// only ever decided by a real gesture, so this band never yanks a reader who
+export const TIMELINE_FOLLOW_REARM_THRESHOLD_PX = 40;
-// is still on the end; what it decides is how close to the live edge a reader
-// who scrolled away must come back before follow re-arms and the pill hides.
-// Half a screen reads as "I am back at the bottom" without having to land on
-// the last pixel, and stray row growth or late measurements cannot push a
-// pinned reader out of it. Distance is measured against the full content
-// length.
-const FOLLOW_REARM_MIN_THRESHOLD_PX = 40;
-export const resolveFollowRearmThresholdPx = (scrollLength: number): number =>
-    Math.max(FOLLOW_REARM_MIN_THRESHOLD_PX, scrollLength / 2);
 
 export const resolveTimelineIsAtEnd = (
     state: {
@@ -90,7 +81,7 @@ export const resolveTimelineIsAtEnd = (
         && typeof scrollLength === 'number'
         && Number.isFinite(contentLength)
     ) {
-        return contentLength - (scroll + scrollLength) <= resolveFollowRearmThresholdPx(scrollLength);
+        return contentLength - (scroll + scrollLength) <= TIMELINE_FOLLOW_REARM_THRESHOLD_PX;
     }
     return state.isNearEnd ?? state.isAtEnd;
 };

packages/ui/src/hooks/useChatTimelineScroll.ts

@@ -9,7 +9,7 @@ import {
     getRowBottom,
     resolveRealContentEndOffset,
     resolveTimelineIsAtEnd,
-    resolveFollowRearmThresholdPx,
+    TIMELINE_FOLLOW_REARM_THRESHOLD_PX,
     type TimelineListMeasurementState,
     type TimelineScrollMode,
 } from '@/components/chat/lib/scroll/timelineScrollAnchoring';
@@ -496,7 +496,7 @@ export const useChatTimelineScroll = ({
                 const lastBottom = lastIndex >= 0 ? getRowBottom(state, lastIndex) : null;
                 if (lastBottom !== null) {
                     const visibleBottom = state.scroll + state.scrollLength - composerOverlayHeightRef.current;
-                    if (lastBottom - visibleBottom > resolveFollowRearmThresholdPx(state.scrollLength)) {
+                    if (lastBottom - visibleBottom > TIMELINE_FOLLOW_REARM_THRESHOLD_PX) {
                         isAtEndRef.current = false;
                         setIsPinned(false);
                         scheduleShowScrollButton();