Comment 10 for bug 1739956

Linux Mint 19.3, closing side panels fixed the issue.

Closed side panels, double clicked on top window bar (don't know the proper name for it, the bar that has the min, resize, max, close buttons on it) and the application window properly resized for monitor size and the resize button appeared again (yay!). Restored the right panel and everything worked fine. Closed app, reopened it, all is good.

Awkward fix but it worked.