Properties panel disappears and won't come back.

This has happened to me a few times today, but I don't have exact details on how to reproduce it. It seems like when I move and resize an object, then try to undo, when the undo has problems then the properties panel disappears and I can't get it back, even by clicking on a different widget.

I *once* got lucky by clicking the property panel, and was able to cause the font size drop down to appear out of nowhere (the list only, not the actual control), but I couldn't see the rest of the control.

It *seemed* to happen more often when I had more mockups open at once, and when many of them had a few custom images. (memory??)

I feel like I'm reporting a Bigfoot sighting more than a bug. Any ideas (even on stuff I might try to reproduce it) would be helpful. Thanks!