Page Manager context menu misbehaving

[CrispinF]

In a site with a lot of pages, the Page Manager started misbehaving. If you need to scroll down to click on the page to work on, the page then automatically scrolls to the top of the menu page and your menu appears in a seemingly random place on the right:

e.g.

and even

It's not certain whether this is only an issue in this site's theme/skin or a widespread issue.