HMMM it's tricky.
The stuck key theory doesn't seem right. Tried pressing them in turn, no success.
BUT!
Interestingly the mouse over the "SPINE" and "ANIMATE" still changes the mouse pointer.

ALSO the window resizing.
It's as if you can resize the window, and it still shifts the point at which you can click and drag the edge of the windows in spine...
but nothing visually changes.

I can't seem to reproduce the bug to happen on cue.
It seems to happen randomly.
I will continue to keep an eye on it!