As shown in the attached screenshot, Assistant and Designer (and possibly other tools that I don't use a lot) frequently show broken rendering. It seems it was happening sometimes when I switch from another active window to one of them.

I notice that when they are made the active window, their window content was refreshed (I can see the blink of the window content area). After the refresh, sometimes it became broken. The content could be brought back by resizing the window, or clicking on the hidden widgets.