Helllo - I have this issue on Ubuntu 16.04 running the JellyFin Theater App. Although I can double click the App and it opens to full-screen it leaves a black side-bar to the left and right top and bottom which stays during the movies so it is not really full-screen.
For example see attached screen shot of running the application with "npm start":

Then here is the example after double-clicking where it goes into semi-full-screen with black edges but crops out part of the application window on left and right:

It would be fine if it would be a true full-screen by double clicking but it cuts out part of the application on left and right along with it cuts out part of the movie on left and right and top and bottom.
Originally posted by @alextomko in #11 (comment)