STR:

1. Start with MOZ_ENABLE_WAYLAND=1 on Ubuntu 23.04.
2. Drag a tab out of the tab strip and over the same window.
3. Press Esc key to cancel drag.

Expected:
Drag is cancelled without dropping.

Actual:
The dragged tab shows the cancel animation but still performs a drop and opens in a new window.

Does not happen with XWayland.

Log created with MOZ_LOG=Widget:5,WidgetDrag:5.

Yes, this is somehow expected. When Wayland D&D ends outside of any window which accepts the D&D the operation is canceled automatically. We use a workaround to create new windows when D&D ends outside of main Firefox window and always create a new window in that case.

I don't think it's actionable unless the D&D protocol is changed somehow. AFAIK Chrome (OS?) has protocol extension for it:
https://bugs.chromium.org/p/chromium/issues/detail?id=1099418

Yeah, I think this would need https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/204