Fix issues with Quick Docs viewer's scroll trapping:
* Need to stopPropagation() or host CodeMirror editor may programmatically
scroll in some cases. (In testing, seems to only happen when making a diagonal
trackpad motion on Mac -- i.e. when wheelDeltaX/Y are both nonzero).
* Don't block scroll events when docs content doesn't need scrollbars - let
host editor scroll naturally in that case.