bug#57468: 29.0.50; Text cursor continues to blink when frame loses focus

[Lars Ingebrigtsen]

Now I'm seeing this phenomenon, too, but I don't have a recipe to
reproduce it either.  But this Emacs has two frames, and if I select one
of the frames, and then select a frame from a different Emacs that
covers this frame, then both the other Emacs and this Emacs has a
blinking cursor.

If I select the frames in a different sequence, then the non-selected
Emacs frame stops blinking.

So it seems like there some sort of interaction between frames being
hidden and frames not noticing that they've lost focus?  But I'm unable
to reproduce this from "emacs -Q".

I've added Po Lu to the CCs; perhaps he has some comments here.  In
particular -- have there been any changes in the code that notices
getting/losing focus lately?