Version 4.5 stops allowing item selection

I thought for sure this would already have been reported, so sorry if I 've missed it. I just updated to v4.5. I can start working on a score and then, at some point, I can no longer select notes, rests or other elements. It sometimes selects nothing and other times selects the measure, even though I am zoomed in and clicking on a note. Even stranger, I can't click on the "X" of the window to close it. I have to choose File, Quit, then when I get back in, it works for a while then the problem returns.