I have figured out the culprit:
The "Screen reader" accessibility setting was turned on for some reason, and if I turn it off, this problem goes away. Steps to reproduce this problem:
1. Turn on screen reader
2. Right click an old email in evolution, choose Edit as new message
3. Paint some text with the mouse, start typing to replace it.
4. Continue trying to type even when the window seemingly freezes.
5. The compose window should go blank and show a view similar to the attached screenshot.
I have figured out the culprit:
The "Screen reader" accessibility setting was turned on for some reason, and if I turn it off, this problem goes away. Steps to reproduce this problem:
1. Turn on screen reader
2. Right click an old email in evolution, choose Edit as new message
3. Paint some text with the mouse, start typing to replace it.
4. Continue trying to type even when the window seemingly freezes.
5. The compose window should go blank and show a view similar to the attached screenshot.