backdrop: make sure users can't scroll behind the backdrop with all overlays

[brandyscarney]

[manucorporat]

I confirm this is a real issue in iOS safari, affecting alerts, popover and actionsheet.

[Manduro]

This was an issue in v1 as well: #4793. The fix for v2 might be similar?

[brandyscarney]

@Manduro thanks for linking to that! :)

[manucorporat]

@brandyscarney @adamdbradley my fix: #6687