Why does my Windows 10 screen turn off by itself while I'm still using the PC?

Why does my Windows 10 monitor randomly turn off while I'm watching a video or playing a game, even though the power plan is set to never turn off the display and never put the PC to sleep? Why does moving the mouse bring it back instantly, but the screen still goes dark again a few minutes later no matter what I do?

Could this be a screen saver kicking in, a display driver timeout, or a USB device triggering the idle timer? And is there a way to find out exactly what's causing the display to turn off while the system is still active?