While trying to export a picture I find the export button at the bottom of the export window is hidden/not visible as the window is extending beyond the screen.
You can resize the window by dragging from the top edge.
Then move it up to reveal the bottom part.
At least this works on Windows, where you can also double-click the top edge to fit the window to screen height.
You have fooled around with the screen resolution making windows not fit on the actual screen area. There was a post a few months ago about something similar and it was some aftermarket program that users was using that caused it.