How do I fix a stretched screen on Mac mini connected to Panasonic TV?

Stretching, compressing, and letterboxing are all artifacts of the Mac sending a video signal that doesn't match the TV's native resolution - the TV is probably doing its best to show the content, but clearly its an imperfect system.

The big question is how you're connecting, and if this is the only display connected to the Mac Mini

If you're connecting via HDMI, then the TV should appear in System Settings -> Displays. You should be able to set a screen resolution that matches your TV

If this is the only display, and you can't actually get to System Settings (because the menu is offscreen), then try rebooting while holding the shift key. This should boot in Safe Mode with default settings, from where you should be able to reset the display.

It sounds like the "stretched screen" is a recent development, correct?

Check your TV settings. You may have accidentally changed the zoom setting in the TV. This is a common thing. \

On the remotes & menus for my Sony TVs there is a "Wide Mode" feature that enables you to change between 4 different zoom settings - Wide, Normal, Full & Wide Zoom - two of which stretch the image. Check your Panasonic TV remote, menus and if necessary the manual.

Congrats! I had a feeling that was the problem. It was on my mind because I often have to adjust the Wide setting depending on the media I watch.