Add host-to-guest file drag and drop

[zats]

This draft only documents my approach. I know the maintainer already plans this feature, so no code review is needed.

Goal

Support host-to-macOS-guest file drag and drop without shared folders.

Approaches tried

• Forward the host drag directly: VZVirtualMachineView has no drag-session bridge.
• Use shared folders: this requires VM configuration and does not meet the goal.
• Start with a fabricated leftMouseDragged: Finder copied before host release.
• Start with a fabricated leftMouseDown: the source panel blocked Finder.
• Move the panel offscreen: AppKit still cancelled the drag without a held mouse button.
• Queue mouseDown through AppKit: the inactive guest helper discarded it.
• Dispatch mouseDown through the panel: the session started but ended immediately.
• Inject mouseDown with Core Graphics: the panel was not yet ready for hit testing.
• Delay Core Graphics injection until the panel is ready: this worked.

Final flow

1. The host VM view accepts the file drag.
2. The host sends file bytes through a virtio socket.
3. Guest Buddy stages the files in its temporary directory.
4. Guest Buddy puts an invisible 48×48 nonactivating panel under the guest pointer.
5. It waits 50 ms for WindowServer hit testing.
6. It injects a Core Graphics leftMouseDown.
7. The panel starts a real NSDraggingSession from mouseDown(with:).
8. The panel moves offscreen so the guest app becomes the target.
9. Host pointer movement becomes guest leftMouseDragged events.
10. Host release becomes a guest leftMouseUp.
11. The guest app accepts or rejects the drop.
12. Guest Buddy reports the real operation to the host.

Workarounds

• Invisible panel: AppKit needs a local view to start the guest drag.
• Core Graphics injection: AppKit has no supported API to start this drag without a local gesture.
• Accessibility permission: Core Graphics input injection requires it.
• 50 ms delay: WindowServer needs time to make the panel hit-testable.
• Main-display mapping: the current coordinate conversion handles one guest display.
• Brief click interception: the panel must receive the initial mouse-down before it moves offscreen.

[insidegui]

Thanks for sharing, this will definitely be useful as a reference!