Unfocused windows stay in front of focused window
since Ubuntu 15 I have a nasty problem using Mate on Thinkpad. Unfortunately I can't figure out what I do to set it off. However the window manager gets scrambled, and the focused window doesn't come to the front. The focused window is in the back of the window stack, so I can use it but can't see much of it -- meanwhile I can't click on the windows in front of it which are "dead". If I minimise the currently focused window, I can begin to click on the other windows, but only in the opposite of the visible order.
It is possible this issue is ThinkPad specific, but again, I regrettably have yet to figure out what starts it.