fix: keep the welcome composer at the bottom

tests/e2e/community-welcome.spec.ts

@@ -65,10 +65,27 @@ for (const width of [375, 768, 1280, 1470]) {
     await expect(page.getByRole('button', { name: '去作品广场逛逛' })).toBeVisible();
     await teachers.getByRole('button', { name: '先自己试试' }).click();
     await heading.scrollIntoViewIfNeeded();
+    const composer = page.getByTestId('coding-message-composer');
+    await expect(composer).toBeInViewport({ ratio: 1 });
+    const box = (await composer.boundingBox())!;
+    expect(850 - box.y - box.height).toBeLessThanOrEqual(40);
+    expect((await page.getByTestId('community-invitation').boundingBox())!.y).toBeLessThan(box.y);
     await page.screenshot({ path: test.info().outputPath(`community-welcome-${width}.png`) });
   });
 }
 
+test('a short window scrolls the welcome while keeping the composer at the bottom', async ({ page }) => {
+  await page.setViewportSize({ width: 375, height: 600 });
+  await page.goto(baseUrl + '?teachers');
+  const composer = page.getByTestId('coding-message-composer');
+  await expect(composer).toBeInViewport({ ratio: 1 });
+  const before = (await composer.boundingBox())!;
+  await page.getByTestId('community-invitation').scrollIntoViewIfNeeded();
+  const after = (await composer.boundingBox())!;
+  expect(after.y).toBeCloseTo(before.y, 0);
+  expect(600 - after.y - after.height).toBeLessThanOrEqual(40);
+});
+
 test('the community link opens only on click and leaves the draft mounted', async ({ page }) => {
   await page.setViewportSize({ width: 1280, height: 850 });
   const communityRequests: string[] = [];

tests/e2e/fixtures/community-welcome.tsx

@@ -35,7 +35,7 @@ export function Layout() {
     <main className="classroom-main flex min-h-0 min-w-0 flex-1 flex-col">
       <div className="classroom-tabs" role="tablist"><button role="tab" aria-selected="true">操作对话</button><button role="tab">作品</button></div>
       <CodingChatBody welcome={!sent} hidden={false} bubble={bubble}>
-        <div className="classroom-chat-view min-h-0 flex-1">{!sent ? <CodingStartHeading /> : <p>第一条消息已发送（测试）</p>}</div>
+        <div className="classroom-chat-view min-h-0 flex-1 flex-col">{!sent ? <><CodingStartHeading /><CommunityInvitation /></> : <p>第一条消息已发送（测试）</p>}</div>
         <div className="classroom-composer-view shrink-0">
           <CodingComposer value={draft} editable canSend={Boolean(draft)} preparing={false} recovering={false}
             runStatus="idle" mode="prompt" queue={{ items: [] }} error={null} recoverableError={false}
@@ -43,7 +43,6 @@ export function Layout() {
             onModeChange={() => {}} onSubmit={() => { setSent(true); setDraft(''); }} onRecover={() => {}}
             onAddFiles={() => {}} onRemoveAttachment={() => {}} />
         </div>
-        {!sent && <CommunityInvitation />}
       </CodingChatBody>
     </main>
   </div>;

tests/e2e/pi-coding-first-chat.spec.ts

@@ -1110,6 +1110,9 @@ test('first PI Conversation is editable under 500 ms and submits before runtime
       await appendFile(performanceFragment, `${JSON.stringify({ editableMs })}\n`);
     }
 
+    await expect(page.getByTestId('coding-message-composer')).toBeInViewport({ ratio: 1 });
+    expect(await page.getByTestId('coding-message-composer').evaluate(el => innerHeight - el.getBoundingClientRect().bottom)).toBeLessThan(40);
+
     const pixelPng = Buffer.from(
       'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=',
       'base64',