Narrow empty-session composer anchoring

Scenario

Open the WebShell composer-layout harness without a session, with mobileWelcomeFooterMiddle enabled and welcome header/footer renderers. At a 760px viewport width, the empty chat pane uses the mobile composer-bottom layout. Resize to 761px and back to 760px.

Checks

• The absolute composer footer aligns with the full chat-pane bottom at 760px.
• The welcome header stays above the composer; the dot field covers the pane, stays behind the foreground, and does not capture pointer events.
• The composer accepts text input.
• At 761px, the mobile absolute-footer rule is inactive; resizing back restores the bottom anchoring.

Evidence

Before the CSS repair, the internal Chromium harness measured a 419px gap between the composer footer bottom and the chat-pane bottom at 760x900. The focused upstream command is:

npx playwright test --config packages/web-shell/playwright.config.ts packages/web-shell/client/e2e/web-shell.smoke.spec.ts --grep 'anchors the empty mobile composer'

After the repair, the focused upstream run passed (3 passed). The embedded extension-host browser verification also passed in Chrome on macOS using the naturally narrow Data Agent sidebar: the welcome content stayed centered, the composer aligned with the sidebar bottom, the dot field remained behind the foreground, and a temporary draft could be entered and cleared without submission.

A baseline dry-run against the global qwen CLI is not applicable to this host-only layout path. The CLI does not render the WebShell DOM or expose the host props mobileWelcomeFooterMiddle, renderWelcomeHeader, and renderWelcomeFooter needed to trigger it. The executable baseline is instead the real Chromium composer-layout harness above, which reproduced the defect as a 419px footer-to-pane bottom gap before the CSS repair.