Currently we have disabled maximising the screen sharing window because we are limiting the max width/height it can take in order to maintain the original aspect ratio. This could be confusing to our users. Instead we could disable the title bar and create a custom one, where the maximising button will just set the max width/height to the window. Something like the following.

As bonus points we could also catch keyboard shortcuts for making a window fullscreen and also set the max width/height.