STR:

In the left branch nav, click on a branch to select it

Use control + click to do a right click on a branch to open the dropdown menu, the branch you have selected stays bold, but the branch above it also gets an outline highlight. If you go select Open Pull Request, it opens a PR for the branch above the one you selected. It can also be a little buggy when just trying to select the branch. 

It seems to work okay if you don't select the branch first, but annoying to keep opening the wrong branch! 

I have a video, but can't seem to find a way to attach it.