fix(web): the composer rests on scroll when a thread opens above its end

apps/web/src/components/chat/timelineScrollAnchoring.test.tsx

@@ -58,6 +58,46 @@ describe("timelineContentOverflowsViewport", () => {
       ),
     ).toBe(false);
   });
+
+  it("counts an unmeasured last row past the viewport as overflowing", () => {
+    // A thread reopened above its end: the tail has an estimated position but no size yet.
+    const unmeasuredTail = [200, Number.NaN];
+    expect(
+      timelineContentOverflowsViewport(
+        buildState({ positions: [0, 900], sizes: unmeasuredTail }),
+        inset,
+      ),
+    ).toBe(true);
+    expect(
+      timelineContentOverflowsViewport(buildState({ positions: [0, 900], sizes: [200] }), inset),
+    ).toBe(true);
+    // A short thread keeps fitting until its last row is measured.
+    expect(
+      timelineContentOverflowsViewport(
+        buildState({ positions: [0, 200], sizes: unmeasuredTail }),
+        inset,
+      ),
+    ).toBe(false);
+    expect(
+      timelineContentOverflowsViewport(
+        buildState({ positions: [0, Number.NaN], sizes: unmeasuredTail }),
+        inset,
+      ),
+    ).toBe(false);
+  });
+
+  it("keeps anchored-turn metrics unknown until the last row is measured", () => {
+    const state = buildState({ positions: [0, 900], sizes: [200] });
+    expect(timelineContentOverflowsViewport(state, inset)).toBe(true);
+    expect(
+      getAnchoredTurnMetrics({
+        state,
+        anchorIndex: 0,
+        composerOverlayHeight: 100,
+        anchorOffset: 24,
+      }),
+    ).toBeNull();
+  });
 });
 
 describe("timeline scroll anchoring", () => {

apps/web/src/components/chat/timelineScrollAnchoring.ts

@@ -77,8 +77,10 @@ export function getRowBottom(state: TimelineListMeasurementState, index: number)
 /**
  * Whether the timeline's real rows extend past the viewport left above the
  * composer. The list's own content length includes the composer inset
- * spacer, so this measures from the last row instead. Unknown row geometry
- * or an unmeasured viewport counts as fitting.
+ * spacer, so this measures from the last row instead. A last row that is
+ * positioned but not yet measured counts as at least 1px tall, so a thread
+ * reopened above its unrendered end still overflows. Otherwise unknown row
+ * geometry or an unmeasured viewport counts as fitting.
  */
 export function timelineContentOverflowsViewport(
   state: TimelineListMeasurementState | undefined,
@@ -91,7 +93,12 @@ export function timelineContentOverflowsViewport(
   if (typeof scrollLength !== "number" || !Number.isFinite(scrollLength) || scrollLength <= 0) {
     return false;
   }
-  const lastBottom = getRowBottom(state, state.data.length - 1);
+  const lastIndex = state.data.length - 1;
+  // Not in getRowBottom: anchored-turn metrics must stay unknown until the row is measured.
+  const lastTop = state.positionAtIndex(lastIndex);
+  const lastBottom =
+    getRowBottom(state, lastIndex) ??
+    (typeof lastTop === "number" && Number.isFinite(lastTop) ? lastTop + 1 : null);
   if (lastBottom === null) {
     return false;
   }