Does anyone have any clue about this? I already searched for it but I did not found anything.
I can drag both windows to resize them. The inner or outer does not matter which one.
For anyone else having this issue please go here: https://github.com/elementary/gala/issues/244
The user Sticcia has a good workaround:
Start an application with the issue like this:
env Exec=env GTK_CSD=0 X
X would be the application e.g.