When resizing a window interactively, we'll set a grab operation and a
grab window, among other things. If we're resizing (including setting
initial size, i.e. mapping) another window, that didn't change position,
don't use the gravity of the grab operation when resizing our own
window.
This fixes an issue with jumpy popup position when moving a previously
mapped gtk popover.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/999
ac01e69a67