I just realized that when typing a post on my mobile phone, there is no way to undo a mistake.
Most commonly I need to undo if I want to move the text around by highlighting, and then doing a cut & paste. This is fiddly on a phone and sometimes I accidentally hit a character on the keyboard and delete the selected text before the cut happens. Then, I have no way to restore the text that disappeared. All I can do is discard/cancel the reply and start again.
I don’t have a problem like this on the desktop because I can hit Control+Z. But not everyone knows you can do this - there should really be undo and redo buttons on the toolbar?
I like the Google Docs implementation: