This may have the same root cause as this one: [URL]

It seems that no touches within a `SliverAppBar` are recognized while scrolling. This includes:
- Back/dismiss buttons
- Action buttons
- Custom gesture detectors in the title widget
- iOS Status bar taps (for the tap to scroll up gesture)

My guess is that the logic that prevents, say, a button within a scroll view from getting pressed if the user taps during scrolling is not distinguishing between slivers that are moving and slivers that are fixed in place.

It seems to me that the proper behavior is that any sliver that is fixed in place ought to be able to receive touch events even while the `CustomScrollView` is scrolling.