It seems that variables `x-gtk-resize-child-frames' and `x-gtk-use-window-move' do not help with this.
I am using KDE with KWin (X11) version 5.27.7. I don't know where there is any known issue on this platform.
From another perspective, is there a way to perform resize and move at the same time?
(I mean, could the two steps be executed within a single redisplay cycle,
so that users would not see the intermediate changes?)
If these two steps are not done separately, the execution order would not matter.
As I wrote in my first email, I can see the child frame being moved and then resized
on my computer, even though the two steps happen very quickly.