Problem: Another clear signal it's a webapp and not a native app, is when the content's scrollbar goes under headers/footers. Only desktop appears to support customizing scrollbar location.
Use Case: Let's say an app has a fixed header/footer, with a transparency effect on the header, and the content area is scrollable. When the content scrolls, it should be able to scroll under the header and footer. All of this works as is, however the scrollbar track is sized to the entire scrollable area, which means the track also shows under the headers and footers, rather than stopping where the header/footer begin.
Proposal: Allow margins to adjust where the scrollbars show.
Current Results:
Below is a tall element so there is a scrollable area. Notice how the scrollbar goes under the header. The scrollbar track can be adjusted on desktop, but not mobile.