This is just a small refactoring to move some trivially common parts of wxScrollHelperBase and wxVarScrollHelperBase in a new common base class. This will make it possible to apply other corrections to wxVarScrollHelperBase without having to physically duplicate the code from wxScrollHelperBase in it. See #15357. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74813 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775