At Try, I entered x and tapped what I presume is Undo. No effect.

Android Chrome.

I ran into some issues on Firefox Android.

I wanted to clear the text box, so I marked the entire text and hit backspace. It only removes the last letter.

So then I marked the entire text and pressed a letter. That successfully replaced all the text with the one letter.

But then the editor broke completely. I cannot put newlines anymore, backspace is inconsistent, and the text cursor isn't shown.

(The editor breaking after marking all the text and pressing a letter is reproducible.)

At least on mobile the editor seems to still be unstable.

Tried to reproduce this, also on Chrome Android, but undo removes the typed character as expected.