This is a follow-up of #1001 and in fact the same issue as https://www.drupal.org/node/687750 against D7 Administration menu...

The only long menu that we currently have in Backdrop (or more correct the only one that is likely to currently occur) is the admin bar search results. #1001 provides an intermediate solution by making sure that the search result entries do not unnecessarily wrap in two lines thus wasting vertical space. Still if either results are too many or the screen height too small, the same issue happens.

The respective issue in d.org was first filed back in 2010 and was not active after August 2011, it saw some action a year later and then again earlier this year in February/March but is still open/unresolved. Among the things/solutions discussed that might be of interest are: 1. The Toolbar Reloaded is a sandbox/poof-of-concept project that was born of and based on an actual user study:

The journey to build a better Drupal toolbar

toolbarv2

d.org issue: https://www.drupal.org/node/1586374 screencast of a live demo: http://screencast.com/t/2CyrTMCUzh 1. Solution For Very Long Dropdown Menus and the respective live demo:

long_menu_scroll

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/1003
GitHub Issue #: 
1003