I have a problem with chat messages shaking, I guess

I made custom message type for attributed text. Message type based on Your text message. Difference is only in field with text (for your message it's text, for my type - attributed text).
I got interesting bug.
When i add new messages - sometimes chat don't whant to scroll them. I found out that chat do some kind of shacking of messages.
Did anybody got same problems?
ThanX

Start by reproducing the shaking and failed scrolling with a custom attributed-text message type, then compare it with the framework's standard text message behavior. Investigate when newly added messages are laid out and scrolled; done means new messages remain stable and the chat scrolls to them reliably.

Written by the indexing model from the issue text.