The browser H5 surface now switches to a phone-oriented shell: the sidebar becomes a closed-by-default drawer, chat stays primary, workspace and terminal panels stay off the mobile chat surface, composer controls use larger touch targets, and mobile menus avoid desktop-only widths and keyboard hints. Desktop and Tauri sidebar behavior remain on the existing store-driven path.
Constraint: H5 is personal/team browser access layered on the existing desktop web UI, so the normal desktop app must keep its current layout behavior.
Rejected: Share the global sidebarOpen default for mobile first paint | it can flash the drawer open before effects run on a phone.
Confidence: high
Scope-risk: moderate
Directive: Keep mobile-only layout branching behind useMobileViewport() && !isTauriRuntime() unless a future task explicitly redesigns the native desktop shell.
Tested: cd desktop && bunx vitest run src/hooks/useMobileViewport.test.tsx src/components/layout/AppShell.test.tsx src/components/layout/Sidebar.test.tsx src/pages/ActiveSession.test.tsx src/components/chat/ChatInput.test.tsx src/components/controls/PermissionModeSelector.test.tsx
Tested: cd desktop && bun run lint
Tested: cd desktop && bun run build
