I've noticed this issue a couple times. It's very common when you're responding to a message chain, but it can happen in any thread where you're responding to the last comment on the page. It has to be somewhere where you've gone past where the side bar ends.

The text area for messages is no longer re-sizable by the browser. Under Old, Chrome would let you drag and resize the text area where you enter comments. Instead under Redesign, the text area auto-sizes. This is... okay... I'd rather have the ability to resize the text area because I know what I want my browser to look like more that the developers, but it does auto-expand as you continue to type and that is functional.

The problem is when the text area is already at the bottom of the screen. See, the text area will expand, but it won't scroll down. You're literally able to type yourself off the bottom of the screen. To see what you're writing, you have to stop typing, scroll the window, and the continue typing. And when you reach the end of that line, you've got to do it again. And again for the next line, and again for the next.

I don't think the window should automatically scroll. Automatic scrolling never works well, because there always seem to be corner cases where the browser doesn't scroll to the right location or when something gets to be too large to display on the screen at once

Instead I think a better solution would be to always have some white space padding at the bottom of the page. This will ensure that the comment editor isn't going to expand itself off the bottom of the browser's display area. Or at least that it won't do so very often.