feat: detachable plugin windows as a bundled Windows plugin

[thomaspblock]

Summary

Desktop windows become browser-like tab containers. Any page (Messages, Projects, Agents, Workflows, external plugins) or launcher panel (Bestie, Agent Activity) can leave the main window into its own OS window, and windows can hold any combination of tabs. Home and Settings stay in main. The feature ships as the bundled Windows plugin (buzz.windows), so it has a toggle in Settings → Plugins like Terminal.

• Move by drag or menu. Drag a tab or launcher out of the strip: past 6px the tab lifts out of its slot into a native always-on-top pill (same size, icon and resolved styles as the tab, follows the app theme) that stays visible across the whole screen. Drop on another Buzz window to merge, anywhere else to open a new window under the pointer; release back in the strip to cancel. Right-click offers Move to new window / main window / Window N. The hovered window highlights where the tab will land (strip for pages, launcher row for panels), is focused on drop, and selects the moved tab.
• One place at a time. A tab lives in exactly one window. Closing a detached window returns its tabs to main; closing main quits. Layout is persisted per profile (<profile>/windows.json) and restored on launch. Disabling or removing a plugin removes its tab wherever it lives; an emptied window shows an explanation with Close window.
• Every window runs the full app, rendering only its assigned tabs. Only main raises desktop notifications; detached windows follow the community selected in main.
• Plugin switch. Rust owns the windowing runtime (src-tauri/src/windows.rs); the new ctx.windows capability exposes the layout to plugins and enable(). The Windows plugin is ctx.effect(() => ctx.windows.enable()). Switching it off hides menus/drag and gathers every tab back into main; a disabled plugin at launch discards the saved layout. App teardown never resets.

Design and decisions: docs/plans/2026-09-14-001-feat-detachable-plugin-windows-plan.md, docs/shell-design.md (Detached tab windows), docs/plugin-architecture.md (Desktop tab windows).

Demo

Video — dragging tabs and Bestie between windows:

windowmagic.mp4

Settings → Plugins with the Windows plugin:

Reviewer notes

• Tabs render the shared NavigationItem pill and launchers the chrome IconButton disc from main; PageTab adds the move menu and pointer drag on top of them. Window commands are generic over the Tauri runtime so they sit in the shared commands() handler.
• FOUNDATION files touched, additively: src/app/services.ts (compose WindowsService, bind notifications only in main), src/plugins/author.ts (export Windows, WindowLayout, WindowSnapshot), src/app/App.tsx.
• tauri.conf.json turns on macOSPrivateApi (with the matching Cargo feature) for the transparent rounded drag pill. Apple rejects that API for App Store submission; bundling is off in this repo, and the fallback is an opaque pill.
• Capabilities: tabs-* windows get the same permissions as main plus event listen/unlisten and window close. The drag-ghost window has no capability and no IPC.
• Not included (documented as later phases): restoring window positions/sizes, reordering tabs within a strip, z-order awareness when windows overlap, moving tabs from plugin code.

Test plan

Automated (all green on the head, macOS): Biome, tsc, design-foundation checks, frontend build, Vitest 1635/1635 (layout filtering, web fallback, drag coalescing, activation, drop-target, lifted-tab ghost, plugin enable/reset/teardown, live toggle in the app integration test), Clippy -D warnings, Rust layout/persistence/reset tests, browser journeys 411/418 in Chromium and WebKit.

• CI: all checks green, including both WebKit browser shards. (Locally on my Mac, tests/browser/emoji.spec.mjs fails on WebKit on main as well — a local environment difference, not this branch.)
• Known local-only: terminal::tests::production_environment_probe fails on machines whose login shell prints a banner; module unchanged.

Manual, desktop build (bin/just desktop):

• Drag Messages out to a new window; drag Agents onto it; close it with the traffic light; both are back in main.
• Drag Bestie onto a detached window: the launcher row highlights, Bestie lands there and opens as the companion; drag it back to main.
• Relaunch: the same distribution of tabs comes back.
• A mention while Messages is detached raises exactly one notification.
• Settings → Plugins → Windows off with two detached windows open: they close, tabs return, no move menu/drag. On again: dragging works.
• Quit with windows detached, disable the plugin, relaunch: nothing restored.
• Multi-display: drop coordinates land on the right screen (deferred check from the plan).

[thomaspblock]

windowmagic.mp4