Mail application gets elongated. And no email is readable due to less vertical space.
To fix this issue you need to kill/close pantheon-mail and then run the command
dconf reset -f /org/pantheon/mail/
I believe this is only a temporary solution. If you have Elementary Tweaks installed you can add that command to Cerbere so is constantly running in the back.
This is a bug. Please subscribe to its ticket in the bug tracker to receive notifications when the devs need help testing a fix.
I had the same issue. I replaced the window and resized it first vertically, which really goes slow. (only some pixels a second) When the window had the correct width again, I could resize it easily to the normal height.