diff --git a/src/client/editor.css b/src/client/editor.css
index c9b59859..79992adc 100644
--- a/src/client/editor.css
+++ b/src/client/editor.css
@@ -839,6 +839,9 @@
   font-weight: 500;
   color: #5b6c4e;
 }
+.document-chat-panel .live-chat {
+  width: 100%;
+}
 .document-chat-panel .chat-persona {
   padding: 18px;
 }
diff --git a/tests/page-chat-layout.test.tsx b/tests/page-chat-layout.test.tsx
new file mode 100644
index 00000000..a1c75e42
--- /dev/null
+++ b/tests/page-chat-layout.test.tsx
@@ -0,0 +1,199 @@
+import { readFileSync } from 'node:fs';
+import { renderToStaticMarkup } from 'react-dom/server';
+import { chromium } from 'playwright';
+import { expect, it, vi } from 'vitest';
+import { Chat } from '../src/client/Chat';
+import { PageReviewCard } from '../src/client/PageReviewCard';
+
+vi.mock('@copilotkit/react-core/v2', () => ({
+  useAgent: () => ({
+    isReady: true,
+    agent: {
+      messages: [
+        { id: 'user', role: 'user', content: 'Please review these notes.' },
+        {
+          id: 'assistant',
+          role: 'assistant',
+          content: 'I prepared a review draft with the key points.',
+          toolCalls: [
+            {
+              id: 'review',
+              type: 'function',
+              function: { name: 'review_space_page', arguments: '{}' },
+            },
+          ],
+        },
+      ],
+    },
+  }),
+  useCopilotKit: () => ({ copilotkit: {} }),
+  useHumanInTheLoop: () => {},
+  useRenderTool: () => {},
+  CopilotChatToolCallsView: () => (
+    <PageReviewCard
+      args={{
+        title: 'Project notes',
+        content: 'A summary of the project plan.',
+        spaceId: 'space',
+      }}
+      status="executing"
+      threadId="thread"
+      toolCallId="review"
+      onSaved={() => {}}
+    />
+  ),
+}));
+vi.mock('../src/client/api', () => ({ api: vi.fn() }));
+vi.mock('../src/client/useVoice', () => ({
+  useVoice: () => ({ status: 'idle' }),
+}));
+vi.mock('../src/client/CallView', () => ({ CallView: () => null }));
+
+async function launchChatBrowser() {
+  try {
+    return await chromium.launch({
+      executablePath: process.env.CHROMIUM_EXECUTABLE_PATH,
+    });
+  } catch (error) {
+    if (
+      process.env.CHROMIUM_EXECUTABLE_PATH ||
+      !(error instanceof Error) ||
+      !error.message.includes("Executable doesn't exist at")
+    ) {
+      throw error;
+    }
+    // Hosted CI images include Chrome even when Playwright's cache is empty.
+    return chromium.launch({ channel: 'chrome' });
+  }
+}
+
+it('falls back to Chrome only when bundled Chromium is missing', async () => {
+  vi.stubEnv('CHROMIUM_EXECUTABLE_PATH', undefined);
+  const browser = {} as Awaited<ReturnType<typeof chromium.launch>>;
+  const launch = vi
+    .spyOn(chromium, 'launch')
+    .mockRejectedValueOnce(
+      new Error("Executable doesn't exist at /cache/chromium"),
+    )
+    .mockResolvedValueOnce(browser);
+  try {
+    expect(await launchChatBrowser()).toBe(browser);
+    expect(launch.mock.calls).toEqual([
+      [{ executablePath: undefined }],
+      [{ channel: 'chrome' }],
+    ]);
+  } finally {
+    launch.mockRestore();
+    vi.unstubAllEnvs();
+  }
+});
+
+it('does not hide other browser launch failures', async () => {
+  vi.stubEnv('CHROMIUM_EXECUTABLE_PATH', undefined);
+  const launch = vi
+    .spyOn(chromium, 'launch')
+    .mockRejectedValue(new Error('browser sandbox failed'));
+  try {
+    await expect(launchChatBrowser()).rejects.toThrow('browser sandbox failed');
+    expect(launch).toHaveBeenCalledTimes(1);
+  } finally {
+    launch.mockRestore();
+    vi.unstubAllEnvs();
+  }
+});
+
+it('does not replace an explicitly configured executable', async () => {
+  vi.stubEnv('CHROMIUM_EXECUTABLE_PATH', '/custom/chrome');
+  const launch = vi
+    .spyOn(chromium, 'launch')
+    .mockRejectedValue(new Error("Executable doesn't exist at /custom/chrome"));
+  try {
+    await expect(launchChatBrowser()).rejects.toThrow(
+      "Executable doesn't exist at /custom/chrome",
+    );
+    expect(launch.mock.calls).toEqual([[{ executablePath: '/custom/chrome' }]]);
+  } finally {
+    launch.mockRestore();
+    vi.unstubAllEnvs();
+  }
+});
+
+it('keeps page chat inside its sidebar across desktop and stacked widths', async () => {
+  const browser = await launchChatBrowser();
+  try {
+    const page = await browser.newPage();
+    const css = ['style.css', 'editor.css']
+      .map((file) =>
+        readFileSync(new URL(`../src/client/${file}`, import.meta.url), 'utf8'),
+      )
+      .join('\n');
+    const html = renderToStaticMarkup(
+      <section
+        className="document-session chat-visible"
+        style={{ height: 900 }}
+      >
+        <div className="document-column">Project notes</div>
+        <div className="document-assistant open">
+          <aside className="document-chat-panel">
+            <div className="document-chat-heading">
+              <span>Page conversation</span>
+              <button>Close</button>
+            </div>
+            <Chat
+              thread={{
+                id: 'thread',
+                dotId: 'dot',
+                ownerId: 'owner',
+                title: 'Page chat',
+                createdAt: 0,
+              }}
+              dot={{
+                id: 'dot',
+                name: 'Research specialist',
+                spaceId: 'space',
+                spaceIds: ['space'],
+                instructions: 'Review notes',
+                researchAllowed: false,
+                memoryAllowed: false,
+                appsAllowed: false,
+                composioToolkits: [],
+                createdAt: 0,
+              }}
+              onConsumed={() => {}}
+              voiceReady={false}
+              calls={[]}
+              paused={false}
+              onSaved={() => {}}
+              onSchedule={() => {}}
+            />
+          </aside>
+        </div>
+      </section>,
+    );
+    await page.setContent(`<style>${css}</style>${html}`);
+    for (const width of [1320, 1150, 901, 900, 700, 390]) {
+      await page.setViewportSize({ width, height: 900 });
+      const bounds = await page
+        .locator('.document-assistant')
+        .evaluate((panel) => {
+          const chat = panel.querySelector('.live-chat')!;
+          return {
+            panelWidth: panel.clientWidth,
+            panelScroll: panel.scrollWidth,
+            chatRight: chat.getBoundingClientRect().right,
+            panelRight: panel.getBoundingClientRect().right,
+          };
+        });
+      expect(
+        bounds.panelScroll,
+        `panel overflow at ${width}px`,
+      ).toBeLessThanOrEqual(bounds.panelWidth + 1);
+      expect(
+        bounds.chatRight,
+        `chat extends outside panel at ${width}px`,
+      ).toBeLessThanOrEqual(bounds.panelRight + 1);
+    }
+  } finally {
+    await browser.close();
+  }
+}, 30000);