workspace: don't activate or restack popup windows
Activating a popup window implicitly causes its parent to be activated as well, which makes it get in the way of the previously active window.
Restacking them below the active window sounds sensible on the surface, but also happens to raise both the popup and its parent above the active window, because the popup is in a different layer than the active window. I would like to change restackWindowUnderActive to be more intuitive, but it looks a bit risky for a bugfix. It can be done later as a refactor.