X.L.MagicFocus: always use focused as master

Instead of searching for the currently focused window across workspaces,
make it so there is never any window above focus on the Stack that is
given to the modified layout.

Closes #657.

CHANGES.md

@@ -37,6 +37,12 @@
     -  Fixed navigation getting "stuck" in certain situations for
        widescreen resolutions.
 
+  * `XMonad.Layout.MagicFocus`
+
+    - The focused window will always be at the master area in the stack being
+      passed onto the modified layout, even when focus leaves the workspace
+      using the modified layout.
+
 ## 0.17.0 (October 27, 2021)
 
 ### Breaking Changes

XMonad/Layout/MagicFocus.hs

@@ -57,14 +57,11 @@ magicFocus = ModifiedLayout MagicFocus
 data MagicFocus a = MagicFocus deriving (Show, Read)
 
 instance LayoutModifier MagicFocus Window where
-  modifyLayout MagicFocus (W.Workspace i l s) r =
+  modifyLayout MagicFocus (W.Workspace i l s) =
-    withWindowSet $ \wset ->
+    runLayout (W.Workspace i l (s >>= Just . shift))
-      runLayout (W.Workspace i l (s >>= \st -> Just $ swap st (W.peek wset))) r
 
-swap :: (Eq a) => W.Stack a -> Maybe a -> W.Stack a
+shift :: (Eq a) => W.Stack a -> W.Stack a
-swap (W.Stack f u d) focused
+shift (W.Stack f u d) = W.Stack f [] (reverse u ++ d)
-    | Just f == focused = W.Stack f [] (reverse u ++ d)
-    | otherwise         = W.Stack f u d
 
 -- | An eventHook that overrides the normal focusFollowsMouse. When the mouse
 -- it moved to another window, that window is replaced as the master, and the