In some applications e.g (Boinc or PlayOnLinux) I have the following bug:bug

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

Where X would be the application e.g. playonlinux.