I am running RStudio Desktop 1.3.959 on Linux Mint 19.3 Tricia.
RStudio Desktop opens in full screen mode only. And I do not see a way how to exit a full screen mode. Alt + Space does not help. The window maximize, resize and move functions are disabled. I can only minimize it. So I have two options only - full screen or minimzed.
The solution is to run RStudio Server locally and open it through a browser.