#99 [osx] zooming in on timeline header fails

[David Mihalcik]

When you zoom in on the timeline header on os x, the
system doesn't redraw the sroll pane correctly. This
causes the header and body to get out of sync; the
effect is cumulative as you zoom in. To fix it, force
the scroll pane to recompute its bounds, then scroll
back to the beginning.