Double-clicking opens folders but not files

When I double-click on a folder, the folder opens. Good.

But when I double-click on a file or document for the first time, it doesn't open (i.e., it doesn't launch the application to read that file). Instead, the filename is shaded and there's a box around the filename as if I had selected the file and then tried to rename the filename. I have to deselect the filename and double-click on it again to open the file.

I figured the mouse clicks were either too fast or too slow, so I tried adjusting double-click speed in the Mouse settings in the Control Panel. But no setting, either fast or slow, fixes the problem.

Any ideas?

I'm running Windows 10 version 1909. I have a Logitech MK345 keyboard/mouse combo.