fix(app): keep the latest turn footer in the chat column on web (#9083)

At widths beyond the chat's max width, the latest turn's footer sat at the
chat's left padding instead of under the message column, while running and
after it ended. Earlier footers were aligned.

#8358 wrapped the web viewport's live auxiliary (the latest turn footer and
pending permissions) in a measuring div with no style. That div is a block
box, so the footer row's alignSelf: center no longer applied. Give it the
same flex column style as the stream rows.

Fixes #9081

packages/app/e2e/browser/agent-turn-end.spec.ts

@@ -276,3 +276,52 @@ test("footer timing fits the chat column and swapping labels keeps controls in p
   await expectFooterWithinWidth(page, 300);
   await expectFooterWithinWidth(page, 880);
 });
+
+async function visibleFooterLeftEdges(page: Page): Promise<number[]> {
+  return page.evaluate(() => {
+    const viewport = Array.from(
+      document.querySelectorAll('[data-testid="agent-chat-scroll"]'),
+    ).find((candidate) => candidate.checkVisibility());
+    return Array.from(viewport?.querySelectorAll('[data-testid="turn-footer"]') ?? [])
+      .filter((footer) => footer.checkVisibility())
+      .map((footer) => Math.round(footer.getBoundingClientRect().left));
+  });
+}
+
+async function expectFootersInOneColumn(page: Page): Promise<void> {
+  await expect
+    .poll(async () => {
+      const edges = await visibleFooterLeftEdges(page);
+      return edges.length >= 2 && new Set(edges).size === 1 ? "one column" : edges;
+    })
+    .toBe("one column");
+}
+
+test("keeps the latest turn's footer in the chat column while it runs and after it ends", async ({
+  page,
+  agent,
+}) => {
+  // Wider than the chat's max width, so the column is centered with space on both sides.
+  await page.setViewportSize({ width: 1600, height: 1000 });
+  const gate = await installDaemonWebSocketGate(page);
+  await openAgentRoute(page, agent);
+  await expectAgentIdle(page);
+
+  gate.holdNextAgentStreamEvent("turn_completed");
+  gate.holdNextAgentUpdate(agent.agentId, "idle");
+  gate.setServerMessageSuppressed("agent_update", true);
+  await submitMessage(page, "emit 1 coalesced agent stream updates");
+  await Promise.all([
+    gate.waitForHeldAgentStreamEvent("turn_completed"),
+    gate.waitForHeldAgentUpdate(agent.agentId, "idle"),
+  ]);
+  await expect(page.getByTestId("turn-working-indicator")).toBeVisible();
+  await expectFootersInOneColumn(page);
+
+  gate.releaseHeldAgentStreamEvent("turn_completed");
+  gate.releaseHeldAgentUpdate(agent.agentId, "idle");
+  gate.setServerMessageSuppressed("agent_update", false);
+  await expectAgentIdle(page);
+  await expect(page.getByTestId("turn-working-indicator")).toHaveCount(0);
+  await expectFootersInOneColumn(page);
+});

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

@@ -82,6 +82,8 @@ const initialFillStyle: CSSProperties = {
   pointerEvents: "none",
 };
 
+// Rows and the live auxiliary (turn footer, pending permissions) center their content with
+// `alignSelf`, which only applies inside a flex container.
 const streamRowStyle: CSSProperties = {
   display: "flex",
   flexDirection: "column",
@@ -1362,7 +1364,9 @@ function WebStreamViewport(props: StreamRenderInput & { isMobileBreakpoint: bool
               ...mountedRows,
             ]}
           </div>
-          <div ref={auxiliaryRef}>{liveAuxiliary}</div>
+          <div ref={auxiliaryRef} style={streamRowStyle}>
+            {liveAuxiliary}
+          </div>
           {shouldRenderEmpty ? listEmptyComponent : null}
         </div>
       </div>