72300 Fixed issue for SliverAppBar collapsedHeight (cla: yes, f: material design, f: scrolling, framework, waiting for tree to go green)

72308 Add ScrollbarTheme/ScrollbarThemeData (cla: yes, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72316 Remove Scrollbar.isAlwaysShown assert based on Scrollbar.controller (a: quality, cla: yes, f: cupertino, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72531 Update Scrollbar behavior for mobile devices (a: fidelity, cla: yes, f: material design, f: scrolling, framework, waiting for tree to go green)