This is quite weird, and I have not figured out how to reproduce it yet, but I bring it up here in case it rings bells, or suggests something that might help.
I created a new .tex file using TeXShop (4.44) on macOS 10.15.5 (Catalina).
After farbling around to get it to compile (long story) I wanted to use the One Window mode.
However, what I find is that if I compile with that mode off, I get the PDF produced. It shows up in the Window menu, and is brought forward after a successful compile.
If I switch to One Window mode from there, the PDF disappears from the Window list, the source window widens as if for the two files (source, preview), but the preview side is empty.
Switching out of One Window mode gives me both source and preview in separate windows, and going back to One Window leaves me in the above state (missing PDF in list and window, although the file itself doesn’t change or go away).
Bells ringing? Thoughts?
This may be very simple, but I am stumped right now.
The pdf disappearing from the Window menu is normal. The tex and pdf should be in a single window, and this window is "represented" in the Window menu by the name of the tex file.
I assume the problem is just that the separator between the tex and pdf panes of the single window has been pushed all the way to the right.
Start dragging from the right edge of the window and it should come up.