Hiding window decorations

[dzaima]

For creating custom popup windows or similar, ability to hide window decorations (also, hiding it from the taskbar) is necessary.

Alternative would be to force everything to use native menus (#193).

For X11, this would involve setting this property with one of a variety of options, e.g.:

Atom type = _NET_WM_WINDOW_TYPE_POPUP_MENU;
XChangeProperty(instance->_windowManager.display,
                instance->_x11Window,
                _NET_WM_WINDOW_TYPE,
                XA_ATOM,
                32,
                PropModeReplace,
                (unsigned char *)&type,
                1);

[dzaima]

Noticed that there's setZOrder and setTitlebarVisible, guided by macOS, which together would handle some of that, but would need a 3rd thing to hide from taskbar. X11 does provide separate _NET_WM_STATE_MODAL to disable decorations (i.e. title bar) and _NET_WM_STATE_SKIP_TASKBAR to hide from taskbar, but doesn't seem to provide anything for just inheriting the z-order from a type.

[Matt Worzala]

There is also apparently https://developer.apple.com/documentation/appkit/nsapplicationactivationpolicy?language=objc for hiding from the dock on macOS. I haven't messed around with it at all yet.

[Luis Thiam-Nye]

What would the cross-platform API look like?

The way it seems to me is that in X11 you have to provide a specific window type, whereas macOS and Windows allow more individual control over window features.
https://docs.microsoft.com/en-us/windows/win32/winmsg/window-styles
https://developer.apple.com/documentation/appkit/nswindowstylemask?language=objc

A possible approach for JWM could be something like setWindowType which takes care of all the necessary details.

[Nikita Prokopov]

Sounds good