fix(app): anchor image growth during timeline reload

Intrinsic image dimensions can arrive after the timeline has painted, outside the React segment-change path. Let the browser preserve the visible content anchor during that resize instead of exposing a frame before the deferred bottom correction.

packages/app/e2e/browser/agent-tab-image-stability.spec.ts

@@ -1,4 +1,4 @@
-import { test as base } from "../support/fixtures";
+import { expect, test as base } from "../support/fixtures";
 import {
   appendSettledTimelineTurns,
   createNearTenMegabyteAssistantPng,
@@ -45,6 +45,131 @@ test("switching between settled agent tabs keeps a real assistant PNG rendered",
   await switchAwayAndBackWithoutImageInstability(page, { image, imageAgent, otherAgent });
 });
 
+test("reloading a timeline anchors near-tail assistant image growth", async ({
+  imageWorkspace: workspace,
+  page,
+}) => {
+  test.setTimeout(120_000);
+  const image = await createSmallAssistantPng(workspace, {
+    alt: "Reload geometry image",
+    fileName: "reload-geometry-image.png",
+  });
+  const imageAgent = await createSettledMockAgent(workspace, "Reload image geometry");
+  await emitSettledAssistantImage(workspace.client, imageAgent, image);
+  await appendSettledTimelineTurns(workspace.client, imageAgent, 3);
+
+  await page.addInitScript(
+    ({ accessibleName }) => {
+      interface ImageGeometryEvent {
+        atMs: number;
+        complete: boolean;
+        height: number;
+        naturalHeight: number;
+        naturalWidth: number;
+        scrollTop: number;
+        width: number;
+      }
+      const state = {
+        accessibleName,
+        events: [] as ImageGeometryEvent[],
+        mutationObserver: null as MutationObserver | null,
+        resizeObserver: null as ResizeObserver | null,
+        target: null as Element | null,
+      };
+      const record = () => {
+        const target = state.target;
+        if (!target) return;
+        const imageElement =
+          target instanceof HTMLImageElement ? target : target.querySelector("img");
+        const scroll = target.closest('[data-testid="agent-chat-scroll"]');
+        const rect = target.getBoundingClientRect();
+        const event: ImageGeometryEvent = {
+          atMs: performance.now(),
+          complete: imageElement?.complete ?? false,
+          height: rect.height,
+          naturalHeight: imageElement?.naturalHeight ?? 0,
+          naturalWidth: imageElement?.naturalWidth ?? 0,
+          scrollTop: scroll instanceof HTMLElement ? scroll.scrollTop : -1,
+          width: rect.width,
+        };
+        const previous = state.events.at(-1);
+        if (
+          !previous ||
+          previous.complete !== event.complete ||
+          previous.height !== event.height ||
+          previous.naturalHeight !== event.naturalHeight ||
+          previous.naturalWidth !== event.naturalWidth ||
+          previous.scrollTop !== event.scrollTop ||
+          previous.width !== event.width
+        ) {
+          state.events.push(event);
+        }
+      };
+      const inspect = () => {
+        const target = Array.from(document.querySelectorAll('[role="img"]')).find(
+          (element) => element.getAttribute("aria-label") === accessibleName,
+        );
+        if (!target || target === state.target) {
+          record();
+          return;
+        }
+        state.target = target;
+        state.resizeObserver?.disconnect();
+        state.resizeObserver = new ResizeObserver(record);
+        state.resizeObserver.observe(target);
+        record();
+      };
+      const start = () => {
+        state.mutationObserver = new MutationObserver(inspect);
+        state.mutationObserver.observe(document.documentElement, {
+          attributes: true,
+          attributeFilter: ["src", "style"],
+          childList: true,
+          subtree: true,
+        });
+        inspect();
+      };
+      if (document.readyState === "loading") {
+        document.addEventListener("DOMContentLoaded", start, { once: true });
+      } else {
+        start();
+      }
+      (
+        window as unknown as {
+          __paseoReloadImageGeometry?: typeof state;
+        }
+      ).__paseoReloadImageGeometry = state;
+    },
+    { accessibleName: image.alt },
+  );
+
+  await openAssistantImageTimeline(page, imageAgent);
+  await expectAssistantImageRendered(page, image);
+  await page.reload();
+  await expectAssistantImageRendered(page, image);
+  await page.waitForTimeout(500);
+
+  const events = await page.evaluate(() => {
+    const state = (
+      window as unknown as {
+        __paseoReloadImageGeometry?: { events: Array<unknown> };
+      }
+    ).__paseoReloadImageGeometry;
+    if (!state) throw new Error("Reload image geometry observer was not installed");
+    return state.events;
+  });
+  const geometryEvents = events as Array<{ height: number; scrollTop: number }>;
+  const initialGeometry = geometryEvents.find((event) => event.height > 0);
+  if (!initialGeometry)
+    throw new Error(`Image geometry was never measurable: ${JSON.stringify(events)}`);
+  const expandedGeometry = geometryEvents.find((event) => event.height > initialGeometry.height);
+  if (!expandedGeometry)
+    throw new Error(`Image geometry never expanded: ${JSON.stringify(events)}`);
+  expect(expandedGeometry.scrollTop, JSON.stringify(events)).toBeGreaterThan(
+    initialGeometry.scrollTop,
+  );
+});
+
 test("a real assistant PNG remains reachable through pagination and remount", async ({
   imageWorkspace: workspace,
   page,

packages/app/src/agent-stream/strategy-web.tsx

@@ -1031,7 +1031,6 @@ function WebStreamViewport(props: StreamRenderInput & { isMobileBreakpoint: bool
       minHeight: 0,
       overflowX: "hidden",
       overflowY: scrollEnabled ? "auto" : "hidden",
-      overflowAnchor: "none",
       overscrollBehaviorY: "contain",
     };
   }, [scrollEnabled]);