I use Wikipedia on mobile via Firefox for Android on a device which has a small screen size (320x452 browser window size).
As the screen size is small, the space I get is limited already. Besides that, the mobile source editor seems to be wasting precious editing space by having lot of unnecessary white space surrounding the editor.
As you could see, effectively the available editing space is only half the screen size! It would be nice if the unnecessary white space was removed used to give more editing space.
I initially thought this was specific to my (small-screen) device. But on checking how the editor appears on a friend's device (a OnePlus 6T; 6.4 inch display) I realised that the situation was even worse on larger screens.
The editing space seems to amount to only a quarter of the available space. Also, it looks rather odd on a device with a large screen.
On further digging into this, it issue seems specific to the Firefox browser. I am unable to reproduce this in Chrome i.e., there is no white space wasted by the editor.
Initially reported in Visual editor for Mobile talk page.