X.H.Rescreen: Allow overriding rescreen itself

The primary motivation is to fix `rescreen` messing up the
workspaces/screens order when making small changes to the layout of
multiple screens — such as resizing virtual monitors via `xrandr
--setmonitor`.

XMonad/Hooks/Rescreen.hs

@@ -15,6 +15,7 @@ module XMonad.Hooks.Rescreen (
     -- $usage
     addAfterRescreenHook,
     addRandrChangeHook,
+    setRescreenWorkspacesHook,
     RescreenConfig(..),
     rescreenHook,
     ) where
@@ -59,16 +60,18 @@ import qualified XMonad.Util.ExtensibleConf as XC
 data RescreenConfig = RescreenConfig
     { afterRescreenHook :: X () -- ^ hook to invoke after 'rescreen'
     , randrChangeHook :: X () -- ^ hook for other randr changes, e.g. (dis)connects
+    , rescreenWorkspacesHook :: Last (X ()) -- ^ hook to invoke instead of 'rescreen'
     }
 
 instance Default RescreenConfig where
     def = RescreenConfig
         { afterRescreenHook = mempty
         , randrChangeHook = mempty
+        , rescreenWorkspacesHook = mempty
         }
 
 instance Semigroup RescreenConfig where
-    RescreenConfig arh rch <> RescreenConfig arh' rch' = RescreenConfig (arh <> arh') (rch <> rch')
+    RescreenConfig arh rch rwh <> RescreenConfig arh' rch' rwh' = RescreenConfig (arh <> arh') (rch <> rch') (rwh <> rwh')
 
 instance Monoid RescreenConfig where
     mempty = def
@@ -89,8 +92,14 @@ instance Monoid RescreenConfig where
 -- 'randrChangeHook' may be used to automatically trigger xrandr (or perhaps
 -- autorandr) when outputs are (dis)connected.
 --
+-- 'rescreenWorkspacesHook' allows tweaking the 'rescreen' implementation,
+-- to change the order workspaces are assigned to physical screens for
+-- example.
+--
 -- Note that 'rescreenHook' is safe to use several times, 'rescreen' is still
--- done just once and hooks are invoked in sequence, also just once.
+-- done just once and hooks are invoked in sequence (except
+-- 'rescreenWorkspacesHook', which has a replace rather than sequence
+-- semantics), also just once.
 rescreenHook :: RescreenConfig -> XConfig l -> XConfig l
 rescreenHook = XC.once $ \c -> c
     { startupHook = startupHook c <> rescreenStartupHook
@@ -104,6 +113,10 @@ addAfterRescreenHook h = rescreenHook def{ afterRescreenHook = userCodeDef () h
 addRandrChangeHook :: X () -> XConfig l -> XConfig l
 addRandrChangeHook h = rescreenHook def{ randrChangeHook = userCodeDef () h }
 
+-- | Shortcut for 'rescreenHook'.
+setRescreenWorkspacesHook :: X () -> XConfig l -> XConfig l
+setRescreenWorkspacesHook h = rescreenHook def{ rescreenWorkspacesHook = pure (catchX h rescreen) }
+
 -- | Startup hook to listen for @RRScreenChangeNotify@ events.
 rescreenStartupHook :: X ()
 rescreenStartupHook = do
@@ -132,7 +145,7 @@ handleEvent e = XC.with $ \RescreenConfig{..} -> do
     -- configuration change, so rescreen and fire rescreenHook. Otherwise,
     -- this is just a connect/disconnect, fire randrChangeHook.
     if ev_event_type e == configureNotify || moreConfigureEvents
-        then rescreen >> afterRescreenHook
+        then fromMaybe rescreen (getLast rescreenWorkspacesHook) >> afterRescreenHook
         else randrChangeHook
 
 -- | Remove all X events of a given window and type from the event queue,