#13 Scroll left/right in timeline moves up

[Mike C. Fletcher]

Scrolling left/right in the timeline seems to cause the
vertical scroll position to get mis-calculated in some
cases. Has been observed both on development and
production machines.

The change is static per-run of the app, but seems to
grow on some machines each time the app is started.
The first screen doesn't seem to show the problem.

Problem is intermittent, but fairly common.

Scrolling up/down with the scrollbar allows access to
the whole timeline, but it's still annoying.