This is an umbrella issue that covers several related issues for work being done on scrollbars.

• Refactor Scrollbar and CupertinoScrollbar to share common methods
  • A couple of different RawScrollbar options have been discussed ([WIP] [Discuss-Only] RawScrollbar (builder version) #70850, [WIP][Discuss-Only] RawScrollbar (painter version) #70798), the later being the current agreed upon direction.
  • We should also fix this while we're at it: "activity?.isScrolling is not true" exception with CupertinoScrollbar #70105
    • This is caused by explicitly creating a drag gesture in CupertinoScrollbar when dragging on the thumb. Instead, we should just update the position. Fix already produced in [WIP][Discuss-Only] RawScrollbar (painter version) #70798
• Support interactive scrollbars #31753: Scrollbars should be fully interactive
  • This includes dragging the thumb, tapping the track
  • CupertinoScrollbar already supports thumb dragging
• Scrollbars should be always visible and instantiated by default on web and desktop #40107: Scrollbars should be created by default for scrollables on Web and Desktop
  • will need to find a way to identify and exclude some cases, like TextFields and PageViews
• Flutter should have a ScrollbarTheme #61362: Scrollbars should have a theme
  • Allows for customization across the app

Initial design doc:
flutter.dev/go/update-scrollbars