It's not in full screen mode.
Nor is the toolbar at the top just in collapsed/minimum mode. I'm unable to bring it back.
When a pdf is loaded, I can expand the the left-side panel and see its vertical toolbar.
When a pdf is not loaded, it's pretty much just a dead application window.
Basic key commands (e.g., Alt+F) do nothing.
It must be some an errant key combination or click that I did somewhere. But — what was it, and how do I undo it?

Thank you!