Solution on DW 9.15.3 and Swift 1.19.0.
Newest Swift update appears to have fixed the issue reported here: https://doc.dynamicweb.com/forum/swift/utilities-sign-in-submenu-display-issue?PID=1605 (where the dropdown menu runs off the screen), but introduced a new issue where the dropdown jumps around: https://www.screencast.com/t/dvT3gnkn2yB .
Assuming it's the same as this bug:
...but if not, just wanted to make sure it got reported.