ScrollView patches

Apologies for the unusually large PR, but much of it was developed last fall as unrelated enhancements on separate `3.x` derived branches, and recently consolidated on to a single branch for integrated testing.

These commits contain bug fixes for [2533078](http://yuilibrary.com/projects/yui3/ticket/2533078), [2533030](http://yuilibrary.com/projects/yui3/ticket/2533030), [2532751](http://yuilibrary.com/projects/yui3/ticket/2532751), as well as additional unit tests and enhancements to existing ones.