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.

BUG: 452955
BUG: 453112