First of all, it seems some git pull --rebase I did from CLI left the repo in some invalid state at some point.

Then I opened up sourcetree and moved my latest changes to the index, thinking that I probably just forgot to commit the previous stuff as it looked like it was still in index.

I clicked the big commit button, and got a dialog saying I've got a partial rebase or something going on in the repo, thinking that ok, this is something I should probably fix I clicked on "Abort" thinking it means "stop whatever is going on right now and let me fix it"

Apparently instead of "abort" the button's real meaning was "CLICK ME AND I WILL DESTROY ALL YOUR WORK, MWAHAHAHA!" as it removed the changes I had just added to the index, from the index AND didn't restore them to the working copy either.

I'm lucky that my IDE can keep history of changes that are applied even outside the IDE so I was able to recover from this.

Either way, it's not cool to destroy people's work like that.