Run sourcetree as usual.
Exit sourcetree.
Right-click on SourceTree shortcut to "run as administrator."
This seems to create or use new settings or environment variables
Notice that SourceTree now fails to find Git or Mercurial on disk
A dialog box appears with 5 options about trying to find git, ignore, browse, or re-download embedded git. None of these options work. Browse doesn't browse. Re-downloading doesn't re-download. Only ignore works. Then git and mercurial not enabled. Preferences/Git shows the button to enable, which presents the same 5 option dialog.

I've tried exiting and running as normal (non-admin). But once this is broken, it seems to stay broken.