Using CSS to center the title actor on the border is a bit ugly, because
it requires the CSS to match the calculations used in chromeHeights().
Also it is not possible to use CSS margins for cases where the position
of the actor is determined at run time, such as for the close button.
Instead use an invisible actor that spans between the horizontal and
vertical center lines of the border as guide when aligning the title
actor.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1313
Start using the new overlays we introduced in the last commit and remove
the WindowOverlay class and the objects for keeping track of them in the
Workspace.
The new layout which doesn't use the -shell-close-overlap CSS property
anymore sligthly changes the position of the close button to be a bit
further away from the actual window.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1298
