Interestingly, after it finished, all menus in Pale Moon had a transparent background. I am using the Maxi4 2.8.2 theme in case that is relevant. Below is what it looked like, I drew the yellow rectangle to make the menu easier to discern.
Restarting Pale Moon resolved the problem. I am not asking for this to be fixed because it is a rare situation and so easily repaired, but I thought it was interesting and looked weird.
