Fix scroll position jumps regression
Commit d37d3ac1 "CompactHistoryScroll: Remove _maxLineCount + 5 lines at a time" caused a regression: addHistLine queried the number of lines in history before and after adding a line, and if it had not incremented it assumed one line entered history and another was dropped at the other end. Now, lines are dropped from history in batches, so take care of that.
Very similar to the regression fixed at 7a1e4768, only this one is about keeping the scroll position on the presence of scroll, and that other one is about keeping the selection.
Thanks to the bug reporters Michael and Luke-Jr for the heads-up.