Describe the bug
On some machines, the sliding animation is jittery.

To Reproduce
Steps to reproduce the behavior:

1. Receive messages
2. It jitters sometimes. (Esp if it's the first message in the frame)

Expected behavior
It should not jitter.

Does the Glass work if you disable all other addons?
Issue still persists even if all other addons are disabled.

Screenshots
A player sent in a video but I'd rather not make that public since it has character names.

Additional context (optional)
This probably caused by how the animations work. There are two separate frames that hold messages, the scroll frame and the sliding frame. Whenever a message (or a group of messages) is received, it's added the sliding frame. Based on the total height of the new messages, the sliding frame is shifted down by that amount. Finally, the sliding frame is animated up to its final position.

Usually this calculation happens so fast that the initial position of the sliding frame is not rendered. However, in some (yet to be determined) scenarios, the messages render before the sliding frame has a chance to shift down.

There are other methods to do the sliding animation, like using SetPoint instead of an animation to do the initial repositioning, so that's worth a shot.

This will be tricky though as most people (including me) don't run into this issue.

Fixed by #71