I'm able to reproduce the issue by opening all (or most) submenus and using intertia scroll to scroll up to top (or bottom) edge of the sidebar panel.
Could you edit the templates/mobile_flat/js/pageinit.min.js file by removing the second instance of the "resize" word, so that the related code is transformed from
...jqm(window).on("pagecontainershow orientationchange resize",function(a){...
to
...jqm(window).on("pagecontainershow orientationchange",function(a){...
Not sure it'll fix the issue, but maybe it will be possible to reproduce the issue in browser's emulator.