As per the docs[1]:
> This parameter is optional for pop-up windows
It _looks_ harmless, but when reparenting the widget (via
swapFrameLoaders or so for DevTools), we keep that association, because
DidChangeParent doesn't fix it up, which is what reorders our windows.
It would seem that the right thing would be to make DidChangeParent also
fix the association (so, remove the mWindowType == Popup check), but
that doesn't work (makes the popup not show up), so just restore
pre-regression behavior.
[1]: https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-createwindowexa
Differential Revision: https://phabricator.services.mozilla.com/D231708
1e34c245a0