> Looks like a Heisenbug.  Sometimes the tooltip is shown for a very short
  > moment only, sometimes it's shown quite normally, sometimes not at all.

I've been able to trace this back to the following commit:

[9d6ec23f7d4f8fbbfdcea353c4b58e47f76a7342] Update frame title when redisplay scrolls selected window

Any objections against the attached patch?

martin