Since a few weeks quite often (but not always!) when I click on a button
which creates a child window (like selection of a file in a directory,
or an alert etc) nothing apparently happens except that I cannot do
anything on the main window.
Actually the child window has opened, but stays behind the main window!
So I have 3 choices: press Alt-F4 (which doesn't always work) or
minimize all windows and then reopen them again (which does'nt always
work either). The 3rd choice is to kill the program!
This may happen in nearly all programs.
I tried to solve this problem with Tweak UI (Prevent application from
stealing focus) but it didn't work.