I just switched to sublime merge away from github desktop (what an improvement), however I just performed the menu option Edit commit\drop selected commits
thinking it was the equivalent of the undo commit
button in github desktop.
That has not undone the commit I had selected (which was the most recent in the tree) as I thought, it has dropped (erased) it completely.
I know there are many posts discussing the reflog
and of course I'll study that but would love a direct (command line if necessary) solution which fixes this exact problem first, then I can learn more later.