LayoutModifier add a modifyLayout

Many layouts are written as layout modifiers because they need to
change the stack of the rectangle before executing doLayout.

This is a major source of bugs. all layout modifiers should be using the
LayoutModifier class. This method (modifyLayout) can be used to
manipulate the rectangle and the stack before running doLayout by the
layout modifier.

XMonad/Layout/LayoutModifier.hs

@@ -33,6 +33,9 @@ import XMonad.StackSet ( Stack )
 -- "XMonad.Layout.Magnifier", "XMonad.Layout.NoBorder",
 
 class (Show (m a), Read (m a)) => LayoutModifier m a where
+    modifyLayout :: (LayoutClass l a) => m a -> l a -> Rectangle
+                 -> Stack a -> X ([(a, Rectangle)], Maybe (l a))
+    modifyLayout _ l r s = doLayout l r s
     handleMess :: m a -> SomeMessage -> X (Maybe (m a))
     handleMess m mess | Just Hide <- fromMessage mess             = doUnhook
                       | Just ReleaseResources <- fromMessage mess = doUnhook
@@ -61,7 +64,7 @@ class (Show (m a), Read (m a)) => LayoutModifier m a where
 
 instance (LayoutModifier m a, LayoutClass l a) => LayoutClass (ModifiedLayout m l) a where
     doLayout (ModifiedLayout m l) r s =
-        do (ws, ml') <- doLayout l r s
+        do (ws, ml') <- modifyLayout m l r s
            (ws', mm') <- redoLayout m r s ws
            let ml'' = case mm' of
                       Just m' -> Just $ (ModifiedLayout m') $ maybe l id ml'