The ☰ button does help them and gives them a single line now, so they are much happier. Example from them here:
There isn’t really much room to work with and it’s surprising that even on newish phones it gives so little vertical space with the keyboard like that. I guess the category / tags could fold up or the GIF/Upload/Full icons flow left more to stop the new line in the future?
Anyway, thanks for looking at it - appreciated!
EDIT: I’ve also reminded the user about the full screen icon that should help here as well 
. It doesn’t look like on mobile you can actually edit in full screen mode but that could be perhaps a future way around this space restriction?