Some info

Note, I haven't tested this in 1.16.1 (only 20w27a) so I'm not sure if you would get the same results there. Also, I couldn't find this same bug reported already, but it was hard to describe this in the search and still get results (so it could be a duplicate).

I unfortunately couldn't get my screen recorder working today so I couldn't attach a video clip like I usually do to this report.

The bug

If a player moves their cursor backwards in a line of the chat or a line of a sign, then when they leave or enter fullscreen mode using the fullscreen key (default F11), then the cursor moves to the beginning of that like of text.

I couldn't confirm this with books and quills (writable books), only signs or chat.

How to reproduce

1. Place a sign so you get to the UI (not an NBT sign) or open the chat
2. Type some stuff
3. Use arrow keys to go backwards in the line of text
4. Toggle fullscreen mode
5. Notice how your cursor has moved to the end of that line of text.