iTerm continues to scroll when looking back in history using the scrollbar. This is undesirable. Once the scrollbar is moved, text must freeze on the screen. This is particularly important when attempting to mark some text (it is very annoying when the text moves during the marking).
The current behaviour seems to try to keep a view of the approximate percentage position, so that the scrollbar shrinks but its centre remains in position, and this causes the view to move.
For example if you are at line 200 and move the view to line 100, then for every two lines added to the end of the text, the text on the screen moves one line.
There is a related issue regarding buffer size.
The size of the scrollback buffer MUST be user settable.
I often need to scroll back thousands of lines. This is a trivial amount of memory on a modern system.
If the system runs out of memory whilst scrolled back the user process must be suspended.
Note: when scrolled back the screen can sometimes be frozen by Ctrl-S. However that also freezes the underlying processes, which is often not desirable.