Is there any way I can detect (in a standard, non-hackish way) whether my Gnome app with a GtkHeaderBar is running under a window manager that provides its own window decorations (for example, XFWM) so that I can set gtk_header_bar_set_show_close_button to FALSE when it's not needed?
UPD: To be more clear, I want to hide GTK header buttons, not the WM ones.
The support for client side decorations should come from a window manager itself or informations about the lack of it from environment. XFWM added support for them in version 4.12.
Oh wow, many commits, such features ! You can read the 4.11.3 announcement here
- Client Side Decoration (CSD) support ! (Compositor need to be enabled). A screenshot with and without on gedit (an application that need CSD) CSD on xfwm4
Source: https://blog.alteroot.org/articles/2015-02-19/new-from-xfce-part-3.html
Moreover, GTK Headerbar should respect content of $GTK_CSD environmental variable and don't show the close button if set to 0 (Please mind that this variable can be not set if the CSDs are supported). As can be seen in Ubuntu.
This means it is something that should be covered for you but you can also try checking for content of $XDG_CURRENT_DESKTOP or read up on more hacky ways to extend support to more WMs and older systems: more in this stack overflow question.
