Fix #887

Viewer's dataSourceDisplay needs to be updated in onTick (which causes it to occur after the clock is ticked but before initializeFrame and render are called.

I tried writing a unit test for this, but it's actually hard to do without coding directly against the implementation.  It's one of those things that's obviously wrong and unlikely to break again (and if it does, I'll do whatever it takes to write a unit test next time).