Chat scrolling is asymmetric for deltas of magnitude less than 1

To reproduce, enable the "Discrete Scrolling" option to guarantee consistent scroll inputs.

1. Run /help to fill the chat
2. Open chat and scroll all the way up
3. Scroll down once
4. Scroll up once
5. Observe that chat position is now one line lower than before

Probably affects other scrollable interfaces as well (the code analysis mentions command suggestions).