xdg_popup.grab in combination with wl_data_device.start_drag can lead to visual artifacts and break popup logic

[vyivel]

Tested in Anvil with 4f9480e

Client: https://gitlab.freedesktop.org/vyivel/randfall/-/blob/main/cases/xdg_popup_interactive_grab_with_dnd.c

Sending xdg_popup.grab followed by wl_data_device.start_drag (done by pressing the left mouse button on the toplevel surface) breaks the popup logic in the following way:

1. The popup can't be dismissed anymore;
2. As a result, keyboard events aren't sent to other clients as the keyboard grab can't be stopped;
3. If another client tries to open a grabbing popup, it's killed with wl_surface@25: error 2: xdg_popup was not created on the topmost popup.

• Side note: the error is sent to the incorrect resource, not_the_topmost_popup is an xdg_wm_base error.

Sending wl_data_device.start_drag followed by xdg_popup.grab (done by pressing the middle mouse button on the toplevel surface) makes the drag icon follow the cursor even after all buttons are released.