This is part of #3956 and shows how your branch has diverged from your target branch (ex: master). This will help you understand what changes can be brought into your branch (behind) and what commits are unique to your branch (ahead).

User Flow to check functionality:

• go to compare view with no toggle showing
• click on compare tab, click into the "compare a branch..." input, and click on "master" and see:
  • the toggle appear in the compare view
  • Behind (x) selected by default where x is the # of commits your branch is behind "master" (old view showed commits ahead of master)
  • the commits that are missing from your branch that exist in the target branch
• click on Ahead (y) in the toggle where y is the # of commits your branch is ahead of "master" and see:
  • the commits that exist only in your branch and not the target branch
• press "x" in the branch input, select another target branch, and see:
  • the compare view where you can toggle behind/ahead
  • the respective commits listed

Changes:

• create toggle
• Behind (x) & Ahead (y) counts
• Behind (x) is selected by default
• default view shows the commits that your branch is behind (relative to the target branch)
• clicking Ahead (y) shows commits
• clicked Ahead (y) shows the commits that your branch is ahead (relative to the target branch)

Other pieces:

• merging into my branch is in [RBB] Merge changes into your branch #3998

Before (shows "ahead" by default):

After: