fix(diagram): don't scroll to bottom when view is scrolled up while recording

This partially reverts commit 817d1524, and adds a should_stick variable to determine whether to enable sticky mode.

This fixes the scenario where the view would scroll to the bottom, even when you're in the middle of the view, when messages come while recording.

Now, the behavior is to dynamically disable/enable is_sticky with respect to is_at_bottom and ignore it if should_stick is disabled.