I wrote a patch[1] for MuPDF's OpenGL viewer (which makes use of GLFW) to enable fullscreen toggle, but XMonad doesn't like GLFW's implementation. Also, since glfwSetWindowTitle sets X11's title, class, and instance to the same value, it's currently impossible[2] to apply a rule for the window, but it's easy to patch and test if you think this can be solved with a manageHook rule. Entering and returning from fullscreen does work correctly with FVWM, Fluxbox, WindowMaker, dwm, Weston, and sway, of which two are Wayland compositors and dwm is a tiling wm like XMonad. What happens with XMonad is that it enters GLFW fullscreen after a multi-second delay and is not able to return from it on toggle. If I patch mupdf to not update the title, add a doFloat rule, and toggle fullscreen, it does not enter fullscreen until I manually interact with the still visibly floating window by trying to move it with a mouse. However, it does return from fullscreen into its original size/position when I press 'f' again. My config is a a personal extension of https://github.com/xmonad/xmonad-testing/blob/master/example-config.hs. Is GLFW's fullscreen toggle implementation generally problematic in XMonad, or can I solve this in my config? As noted in [1], llpp fullscreen toggle works with XMonad, and it uses OpenGL as well, but it doesn't rely on GLFW, so this isn't a general issue with OpenGL but specific to GLFW. [1] https://bugs.ghostscript.com/show_bug.cgi?id=697593 [2] https://bugs.ghostscript.com/show_bug.cgi?id=697634