Browser top and bottom insets incorrect (especially since iOS 11)

[defagos]

The top and bottom browser insets are incorrect:

• Starting with iOS 11, and depending on viewfit-cover (see this article), the behavior needs to be adjusted differently. Check with www.apple.com, for example.
• The bottom toolbar covers the content (it does not count in the safe area / layout guides).

Fixing these issues requires some modernization layout work (to deal with the many iOS 11 changes mostly). I already have implementations bits around, but they should be brought together first.