#### (plus some more docs)

Fixes https://github.com/flutter/flutter/issues/76760
Fixes https://github.com/flutter/flutter/issues/82391
Fixes https://github.com/flutter/flutter/issues/45619
Fixes #117316
Fixes #110956
Fixes #127282 
Fixes #32563
Fixes #46089 
Fixes #79077
Part of fixing #62833

This fixes (a bunch of) issues that have been reported differently over the years, but all have the same root cause. Sometimes the NestedScrollView would incorrectly calculate whether or not there is enough content to allow scrolling. This would only apply to drag scrolling (not mouse wheel scrolling for example). This did not relate to how the extent of the NestedScrollView is computed, but just the logic that enabled the actual drag gestures. This fixes that. :)