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Hiroshi paints its own chrome across the whole window and never shows the native OS titlebar. titlebarTransparent: true in the render() call is what does it, and it is not conditional on the platform: a native titlebar on Windows would be a second titlebar on top of the first.
Do not reintroduce titlebarTransparent: process.platform !== 'win32'. It gives Windows back its own caption, which is exactly what the custom chrome avoids.

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Dragging

On Windows, there is no caption to grab, so the app moves the window itself. The press records where the window and the cursor are, every move pushes the window by the cursor delta, and the release lets go.
Three things are needed on a drag region:
  1. pointerEvents: 'auto'. An unfilled in-flow div gets no GPUI hitbox at all, so without it the press never reaches the handler.
  2. All three handlers on the same element. GPUIX arms the pointer capture on the press only when the element also listens for moves.
  3. A region that does not cover the buttons. The title text and the empty space drag; the controls do not.
The drag lives in src/window-drag.ts. It resolves the window handle from the foreground window, verifies the process id, and moves the window with SetWindowPos. A release that lands outside the window never reaches the titlebar, so the move handler checks the button state and ends the drag itself. On macOS, the app retains the original AppKit mouse-down event before GPUI delivers it to JavaScript. A titlebar press must match that event’s window and coordinates while the left button is still held. Stale, repeated and unrelated requests fail. Tracking is queued on the Cocoa main thread through performWindowDragWithEvent:, so JavaScript does not wait inside the native drag loop. AppKit owns subsequent movement and release; native traffic lights and Spaces remain under OS control. The same handler covers the conversation, sidebar, surface and settings titlebars. On Linux, dragging the custom titlebar is deliberately disabled. GPUIX 0.11.0 does not expose the compositor move request/Wayland input serial needed to do this correctly. Use your desktop’s Move Window menu action or configured shortcut. General settings shows this limitation. Native chrome remains hidden on all platforms; do not change titlebarTransparent to work around it. The browser preview has no native window and does not drag the browser.

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Development and verification

bun run dev builds the macOS Cocoa helper before starting the source window. If running bun app.tsx directly on macOS, first run bun scripts/build-window-macos.ts. The packaged app carries the same helper beside its executable. GPU hit tests cover all four drag regions and existing double-click behavior on Windows/macOS. bun scripts/test-window-macos.ts compiles and runs the production Objective-C bridge against controlled window/event objects, verifying event identity, flipped coordinates, ownership, release races, stale requests and asynchronous dispatch. It does not generate physical mouse input or claim to measure window movement on a hosted macOS runner. The OS tracking call itself is AppKit’s native implementation. Windows real-window integration remains in CI.

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Maximize, restore, and the double click

The maximize button and a double click on the titlebar share one toggle.
The native zoom only maximizes on Windows, it never restores. The restore goes through ShowWindow(SW_RESTORE), which also clears the zoomed state. A plain window move restores the rectangle but leaves IsZoomed true, so the next click would restore again instead of maximizing.
The double click is detected from the timestamps and the positions of two presses, because GPUIX <div /> elements have no onDoubleClick.

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Why Windows does not use the OS move loop

The obvious way to drag a custom titlebar is to hand the drag to the OS modal move loop:
Both are dropped by the GPUI Windows backend. Measured against a live Hiroshi window: the press arrives, the handle resolves, the call returns, and the window moves zero pixels. So the app moves the window itself.
GPUIX exposes no window move API. The native renderer has minimizeWindow, zoomWindow, toggleFullscreen, activateWindow, setWindowTitle and getWindowSize, and nothing to move a window with. A startWindowDrag or setWindowPosition in packages/native would take the FFI out of every app.