fix(app): stabilize session timeline layout continuity (#34533)

packages/app/e2e/regression/session-timeline-accessibility.spec.ts

@@ -0,0 +1,22 @@
+import { expect, test } from "@playwright/test"
+import { assistantMessage, setupTimeline, shell, userMessage } from "../performance/timeline-stability/fixture"
+
+test("space activates a focused timeline button instead of scrolling", async ({ page }) => {
+  const shellID = "prt_space_button_shell"
+  await setupTimeline(page, {
+    messages: [userMessage(), assistantMessage([shell(shellID, "completed", lines(5))])],
+    settings: { shellToolPartsExpanded: false },
+    reducedMotion: true,
+  })
+  const scroller = page.locator(".scroll-view__viewport", { has: page.locator("[data-timeline-row]") })
+  const trigger = page.locator(`[data-timeline-part-id="${shellID}"] [data-slot="collapsible-trigger"]`)
+  await trigger.focus()
+  const before = await scroller.evaluate((element) => element.scrollTop)
+  await trigger.press("Space")
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  expect(await scroller.evaluate((element) => element.scrollTop)).toBe(before)
+})
+
+function lines(count: number) {
+  return Array.from({ length: count }, (_, index) => `line ${index + 1}`).join("\n")
+}