QuickTime screen recording broken

I have been asked to make a screen recording using QuickTimeX.

The feature seems broken in my user account. The little black window appears, but I can't start the recording and the dropdown menu is not available. QuickTime appears to behave oddly compared to other apps I've used. Sometimes its windows don't react to mouse clicks and the mouse click will "pass through" the QuickTimeX window and "hit" the desktop, causing the Finder's focus to switch to Finder.

I booted to Recovery Partition and repaired permissions and did Disk First Aid repair disk. Each claimed to accomplish something.

I booted to an external volume and ran DiskWarrior 4.4. It claimed to do some things that looked minor.

I booted to Recovery Partition and reinstalled OS X 10.9.5 on top on my current installation, not by "nuke and pave."

Still no screen recording function. I quit QuickTimeX, found its 4 prefs in ~/Library/Preferences and put them in a folder, zipped the folder, and then saved the .zip and threw away the folder & its contents. Even though QuickTimeX was not running, I could not empty the trash of one of the prefs without using Terminal.

I made a new admin user. In that account, QuickTime PlayerX screen recording works fine.

I assume I'm overlooking something obvious and simple. What might it be?