X.H.EwmhDesktops: optionally advertise fullscreen support in _NET_SUPPORTED

Provide a way to advertise _NET_WM_STATE_FULLSCREEN in _NET_SUPPORTED.
I had this hardcoded for a while but read the mpv manpage recently:

    Specifically, yes will force use of NetWM fullscreen support, even
    if not advertised by the WM. This can be useful for WMs that are
    broken on purpose, like XMonad. (XMonad supposedly doesn't advertise
    fullscreen support, because Flash uses it. Apparently, applications
    which want to use fullscreen anyway are supposed to either ignore
    the NetWM support hints, or provide a workaround. Shame on XMonad
    for deliberately breaking X protocols (as if X isn't bad enough
    already).

I'm not sure Flash is still a good reason these days to not advertise
fullscreen support yet still handle it and look like idiots while doing
so. (And if anyone thinks it is a good reason, their unchanged xmonad
config will continue behaving that way.)

CHANGES.md

@@ -4,6 +4,13 @@
 
 ### Breaking Changes
 
+  * `XMonad.Hooks.EwmhDesktops`
+
+    It is no longer recommended to use `fullscreenEventHook` directly.
+    Instead, use `ewmhFullscreen` which additionally advertises fullscreen
+    support in `_NET_SUPPORTED` and fixes fullscreening of applications that
+    explicitly check it, e.g. mupdf-gl, sxiv, …
+
 ### New Modules
 
   * `XMonad.Layout.TallMastersCombo`

XMonad/Hooks/EwmhDesktops.hs

@@ -21,7 +21,9 @@ module XMonad.Hooks.EwmhDesktops (
     ewmhDesktopsLogHookCustom,
     ewmhDesktopsEventHook,
     ewmhDesktopsEventHookCustom,
-    fullscreenEventHook
+    ewmhFullscreen,
+    fullscreenEventHook,
+    fullscreenStartup
     ) where
 
 import Codec.Binary.UTF8.String (encode)
@@ -47,19 +49,20 @@ import XMonad.Util.WindowProperties (getProp32)
 -- > import XMonad
 -- > import XMonad.Hooks.EwmhDesktops
 -- >
--- > main = xmonad $ ewmh def{ handleEventHook =
--- >            handleEventHook def <+> fullscreenEventHook }
+-- > main = xmonad $ ewmhFullscreen $ ewmh def
+--
+-- or, if fullscreen handling is not desired, just
+--
+-- > main = xmonad $ ewmh def
 --
 -- You may also be interested in 'docks' from "XMonad.Hooks.ManageDocks".
 
 
 -- | Add EWMH functionality to the given config.  See above for an example.
 ewmh :: XConfig a -> XConfig a
-ewmh c = c { startupHook     = startupHook c +++ ewmhDesktopsStartup
-           , handleEventHook = handleEventHook c +++ ewmhDesktopsEventHook
-           , logHook         = logHook c +++ ewmhDesktopsLogHook }
- -- @@@ will fix this correctly later with the rewrite
- where x +++ y = mappend y x
+ewmh c = c { startupHook     = ewmhDesktopsStartup <+> startupHook c
+           , handleEventHook = ewmhDesktopsEventHook <+> handleEventHook c
+           , logHook         = ewmhDesktopsLogHook <+> logHook c }
 
 -- |
 -- Initializes EwmhDesktops and advertises EWMH support to the X
@@ -213,6 +216,23 @@ handle f (ClientMessageEvent {
           return ()
 handle _ _ = return ()
 
+-- | Add EWMH fullscreen functionality to the given config.
+--
+-- This must be applied after 'ewmh', like so:
+--
+-- > main = xmonad $ ewmhFullscreen $ ewmh def
+--
+-- NOT:
+--
+-- > main = xmonad $ ewmh $ ewmhFullscreen def
+ewmhFullscreen :: XConfig a -> XConfig a
+ewmhFullscreen c = c { startupHook     = startupHook c <+> fullscreenStartup
+                     , handleEventHook = handleEventHook c <+> fullscreenEventHook }
+
+-- | Advertises EWMH fullscreen support to the X server.
+fullscreenStartup :: X ()
+fullscreenStartup = setFullscreenSupported
+
 -- |
 -- An event hook to handle applications that wish to fullscreen using the
 -- _NET_WM_STATE protocol. This includes users of the gtk_window_fullscreen()
@@ -298,6 +318,20 @@ setSupported = withDisplay $ \dpy -> do
 
     setWMName "xmonad"
 
+-- TODO: use in SetWMName, UrgencyHook
+addSupported :: [String] -> X ()
+addSupported props = withDisplay $ \dpy -> do
+    r <- asks theRoot
+    a <- getAtom "_NET_SUPPORTED"
+    fs <- getAtom "_NET_WM_STATE_FULLSCREEN"
+    newSupportedList <- mapM (fmap fromIntegral . getAtom) props
+    io $ do
+        supportedList <- fmap (join . maybeToList) $ getWindowProperty32 dpy a r
+        changeProperty32 dpy r a aTOM propModeReplace (nub $ newSupportedList ++ supportedList)
+
+setFullscreenSupported :: X ()
+setFullscreenSupported = addSupported ["_NET_WM_STATE", "_NET_WM_STATE_FULLSCREEN"]
+
 setActiveWindow :: Window -> X ()
 setActiveWindow w = withDisplay $ \dpy -> do
     r <- asks theRoot