The feature request
The current Github desktop, doesn't feature forked repo branches which is not PR'ed, and the solution for this is to manually git remote add forked repo, see the forked repo branches, and handle merge/rebase into your own branch manually.
Proposed solution
Integrate a "Fork Remotes" UI panel after "Pull Request", which allows users to remotely add forked repos, track un-PR'd branches, and run merge or rebase operations directly within the GUI.
Additional context
Fetch branches from forked repo and display it on GUI, then user will select specific branch, after that display a confirmation modal whether to add the branch into the your repo or cancel the action. Upon confirmation, fetch the branch from the fork URL and track it in the local repo then append the newly tracked branch to the "Other branches" list.

The feature request
The current Github desktop, doesn't feature forked repo branches which is not PR'ed, and the solution for this is to manually
git remote addforked repo, see the forked repo branches, and handle merge/rebase into your own branch manually.Proposed solution
Integrate a "Fork Remotes" UI panel after "Pull Request", which allows users to remotely add forked repos, track un-PR'd branches, and run merge or rebase operations directly within the GUI.
Additional context
Fetch branches from forked repo and display it on GUI, then user will select specific branch, after that display a confirmation modal whether to add the branch into the your repo or cancel the action. Upon confirmation, fetch the branch from the fork URL and track it in the local repo then append the newly tracked branch to the "Other branches" list.