Apps won't "activate" - haunted by phantom app?

[Pw1exclam]

I just installed 18.3 Cinnamon on an Acer laptop, and some phantom app seems to be haunting me. Here are the symptoms:

1) When I hook my laptop up to my TV via HDMI and run VLC to play videos, VLC cannot stay active. By that I mean, if I want to press N on my keyboard to skip to the next video, I can't. I have to Alt-Tab to return control to VLC, and then hit N. Five seconds later, VLC loses control again, and once again I have to hit Alt-Tab. I thought it might be some weird HDMI issue, but the next symptom happens even when the TV is not connected.

2) After hitting Ctrl-Alt-End, I cannot simply hit Enter to shut down. The icon in the taskbar is blinking red, indicating I have to click on it first to activate the dialog box. I cannot even Alt-Tab to it - I have to use a touchpad or mouse to get there.

What on earth could be causing this? Thanks for any help.