OK, here's the slightest failing I've felt numerous times: Old laptop whose graphics under WinXP were just fine, if a little slow. Install Linux with your favorite desktop (LXDE works best for me on that machine). Machine is under moderate load. Click and drag a window to move it. When you release the mouse button, the window is still following the cursor, because the mouse-button-up event was not handled properly.
JWZ complained abut this sort of thing (events not being presented to the handler in proper order) when he was working on the Unix/X version of Mozilla. ISTR him saying that it was a problem wih the protocol itself, which would explain why the problem persists almost 20 years alter.