Windows 10 focus corruption

Question

I have a Windows 10 system that I use both directly and as the display for a clutch of headless servers over VNC. I have worked with this arrangement since Windows 7, without any issues arising. Since, as far as I can tell, some recent updates, focus management in both direct use and in communication with my servers gets corrupted over a period of minutes. Buttons on the taskbar do not respond, shortcuts on the desktop fail to open, focus on xterms over VNC becomes erratic and bizarre. The (temporary) cure is to open Task Manager. After Ctrl-Alt-Del the Task Manager option does not change to the correct cursor until after a first click. A second click brings Task Manager into focus, and all is well, but only for a few minutes. Clicking on the Task Manager icon on the taskbar does not help - it has no effect. Rollover popups do not work.

As I have been typing this it has happened again. No problem with typing this message, but unable to switch focus. I haven't found a way of provoking the problem - it seems to be more a matter of time than of activity - but the way it manifests itself and the way to get temporary relief are in a sense stable. I encountered this maybe 50 times yesterday.

What's going on?