When a tab is dragged outside its current notebook, the drag is terminated completely as soon as the pointer leaves.

The leave can be caused by it entering another widget, seemingly either up or down the stack, or by leaving the notebook area. As our original reporting user describes it:

> 3. As soon as the cursor leaves tab bar, the drag is literally thrown away (blank 'drag' block slides rapidly back to original location)

I can confirm that this is a regression from 3.20.3 to 3.20.4. It is reproducible with some very minimal test code written in Python.

Downstream report: https://github.com/mypaint/mypaint/issues/682
Test code: https://gist.github.com/achadwick/1b3505beb47d65fedfd6

For our test code, it looks like when the drag pointer leaves the notebook itself into the packed page widget (a GtkLabel), the drag is terminated almost as if the user had pressed escape (but with an animation). The same is true when it leaves the toplevel window.

If you are *very* quick in dragging the tab outside the toplevel and onto the desktop, you can sometimes overcome this. The pointer can then move back over any of the application's windows, and drags between notebooks are possible again.

Dragging tabs between notebooks is an important part of GIMP's and MyPaint's UX, so we need this to work.