You can drag the cupertinoscrollbar if you pass an active scrollcontroller to the scrollbar.

Cupertino's thumb would jump up/down in size at the overscroll boundary, and this fixes it.

- Fixed extentInside calculation in ScrollMetrics
- Added asserts to extentInside getter, as well as ScrollPosition.applyContentDimensions to enforce minScrollExtent <= maxScrollExtent
- Added padding to ScrollbarPainter, updated implementation. Took care of some edge cases.
- Changed some scroll bar constants on Cupertino side.

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Add minOverscrollLength to make CupertinoScrollbar thumb correct size in long scroll views.

It was causing problems rolling Flutter into Fuchsia

* Create CupertinoScrollbar

* handle main axis margin

* Adaptive material scrollbar and tests

* Small tweaks

* reapply changes on head

* Docs

* start

* Refactored ScrollbarPainter to be more immutable

* fix tests

* fix bug: one animationcontroller pointed to multiple painters

* some docs tweak

* remove unused import

* review

* review

* add dispose