Fullscreen vs. reading mode

[Daniel]

If I see it correctly, there is no distinction in LyX between the window 
being in fullscreen mode and in reading mode. Here is what I mean:

When one clicks on View > Fullscreen, the LyX window is set to full 
screen and also certain UI elements are hidden (as defined in the 
preferences). This is what I call a (fullscreen) "read mode". On the 
other hand, there is a fullscreen functionality that some window 
managers support, e.g. the one of macOS. This is what I call a 
"fullscreen mode".

However, LyX treats both the same. If one enacts the fullscreen mode, 
one gets the reading mode. (There is actually some bug there because one 
gets only a partial reading mode: only when opening a new tab do the 
tabs and scrollbars disappear but the toolbars stay.)

I think it would be good to distinguish these two, so that one can 
either activate the reading mode which will make sure that the window is 
in fullscreen and the UI elements hidden or use the window manager's 
fullscreen functionality which will just allow one to work as usual but 
in fullscreen.

Or maybe fullscreen functionality should be completely distinguished 
from the read mode. So one can choose either of them separately from the 
menu, e.g. one can even hide UI elements without entering fullscreen mode.

I'd be happy to take a look at the implementation, if there are no 
objections. But please object if you see things differently.

Daniel