Hey, it’s possible that your browser font size for Twitch is changed away from 100%. We have a bug right now where that will often push the bottom chat buttons onto two lines, which in turn expands the input area to cover up some of the bottom part of chat.
Try pressing Ctrl+0 to reset it. Ctrl + / Ctrl - also work, as well as Ctrl + scroll wheel.
This should fix your issue, but again, it’s our fault that it’s happening (you should be able to use that browser functionality without any adverse effects). Fixing this is on our todo list.