Add the support of a title bar region for window dragging

[tasuren]

This is related to #4947.

Problem

The GPUI window title bar height is fixed at the default height of 28px on macOS. It seems that we cannot make it any longer than 28px, which limits UI customization for applications that require a taller draggable area.

Proposed Solution

I propose adding a drag_region method to the Div component, which enables window dragging. This is inspired by the data-tauri-drag-region approach used in Tauri.

This method would allow developers to designate arbitrary areas as draggable, thereby decoupling window dragging from the fixed title bar.

Alternatively, instead of introducing drag_region, we could add a start_window_drag method to the window itself, which can then be invoked from an on_mouse_down event. This approach may require fewer changes overall. This approach is similar to winit’s drag_window method.

Pros

• Allows for a taller title bar area.
• Aligns with patterns from similar frameworks like Tauri.

[tasuren]

I tried to addressed this problem by adding start_window_drag method in my fork.  
Currently, my implementation only supports macOS. If this solution is acceptable, I’m happy to implement it for Windows and Linux as well.

2025-05-24.11.30.23.mov

[mikayla-maki]

@tasuren We would be happy to use this kind of approach. However, we'd prefer it if you re-used the existing Platform::start_window_move API that is currently implemented on Wayland :)

[tasuren]

Thank you! I've updated the code to re-use start_window_move and committed the changes.