Broadcast DestroyWindowEvent to layouts

Some layout and layout modifiers that keep track of some window
properties don't do garbage collection and update their state when
windows are destroyed. By broadcasting this event, it should be easier
for layouts to clean up

Related: https://github.com/xmonad/xmonad-contrib/pull/474

CHANGES.md

@@ -50,6 +50,9 @@
 
   * Document the help command in the help message.
 
+  * `DestroyWindowEvent` is now broadcasted to layouts to let them know
+    window-specific resources can be discarded.
+
 ## 0.15 (September 30, 2018)
 
   * Reimplement `sendMessage` to deal properly with windowset changes made

src/XMonad/Layout.hs

@@ -27,6 +27,7 @@ module XMonad.Layout (
 import XMonad.Core
 
 import Graphics.X11 (Rectangle(..))
+import Graphics.X11.Xlib.Extras ( Event(DestroyWindowEvent) )
 import qualified XMonad.StackSet as W
 import Control.Arrow ((***), second)
 import Control.Monad
@@ -230,6 +231,9 @@ instance (LayoutClass l a, LayoutClass r a) => LayoutClass (Choose l r) a where
     handleMessage c@(Choose d l r) m | Just ReleaseResources <- fromMessage m =
         join $ liftM2 (choose c d) (handle l ReleaseResources) (handle r ReleaseResources)
 
+    handleMessage c@(Choose d l r) m | Just e@DestroyWindowEvent{} <- fromMessage m =
+        join $ liftM2 (choose c d) (handle l e) (handle r e)
+
     handleMessage c@(Choose d l r) m | Just (JumpToLayout desc) <- fromMessage m = do
         ml <- handleMessage l m
         mr <- handleMessage r m

src/XMonad/Main.hs

@@ -315,10 +315,15 @@ handle (MapRequestEvent    {ev_window = w}) = withDisplay $ \dpy -> do
 
 -- window destroyed, unmanage it
 -- window gone,      unmanage it
-handle (DestroyWindowEvent {ev_window = w}) = whenX (isClient w) $ do
+-- broadcast to layouts
+handle e@(DestroyWindowEvent {ev_window = w}) = do
+  whenX (isClient w) $ do
     unmanage w
     modify (\s -> s { mapped       = S.delete w (mapped s)
                     , waitingUnmap = M.delete w (waitingUnmap s)})
+  -- the window is already unmanged, but we broadcast the event to all layouts
+  -- to trigger garbage-collection in case they hold window-specific resources
+  broadcastMessage e
 
 -- We track expected unmap events in waitingUnmap.  We ignore this event unless
 -- it is synthetic or we are not expecting an unmap notification from a window.