On "compare view" I can select a bookmark from the dropdown on a fork, but I can't create a pull request from that bookmark. Going to the pull request UI, I can create a pull request from a changeset that happens to be a bookmark, but there's no bookmark section in the dropdown – only named branches.

It would be great to have bookmarks in the Pull Request dropdown and to be able to create a Pull Request from a bookmark in the compare view. Since these are somewhat analogous to "feature branches" it would also be nice to be able to close/delete them on PR, like branches.