The Discard Changes window appears to allow some form of staging/unstaging of changes. It's inconsistent in that while it allows you to stage/unstage hunks:

.. it doesn't allow you to stage/unstage individual files, but does allow you to stage/unstage all files (note the disabled checkbox for the file but enabled checkbox for "Unstaged files"):

Clicking the "Unstaged Files" checkbox will stage all of the files and then close the Discard Changes window. Staging changes doesn't actually show them in the "staged files" section either.

Additionally, there is an unlabelled checkbox above the list of staged files next to the sorting options:

I find it weird that the Discard Changes window allows staging/unstaging in the first place. Since changes can be discarded individually from the File Status view anyway, it would make more sense if the Discard button merely offered the Reset All operation.