X.L.ResizableThreeColumns: Fix bottom right window not resizing

...by introducing yet another special case.  Unsurprisingly, this is
just a bandaid—the logic in this module is just wrong.  It
purposefully (not introduced with this change) computes the wrong
positions for certain extreme points such that splitVertically (which
simply goes down the list of stack windows _in order_) still works.

What we should do instead is to either

  - keep track of windows that want to change their size and compute a
    rectangle for them first, or

  - immediately when handling the resize message, compute *all* of the
    new sizes immediately (instead of only for the window getting
    resized).

The latter would force us to keep track of the size of the current stack
we operate in, but since 'handleMessage' lives in X this should not pose
a big problem.  I reckon this is the better approach.

Fixes: https://github.com/xmonad/xmonad-contrib/issues/788

XMonad/Layout/ResizableThreeColumns.hs

@@ -99,8 +99,13 @@ instance LayoutClass ResizableThreeCol a where
       mresize s MirrorExpand = mresize' s (negate delta)
       mresize' s delt =
         let up = length $ W.up s
-            total = up + length (W.down s) + 1
+            down = length $ W.down s
-            pos = if up == (nmaster-1) || up == (total-1) then up-1 else up
+            total = up + down + 1
+            pos = if up == nmaster - 1           -- upper right
+                  || up == total - 1             -- upper left
+                  || up `elem` [down, down + 1]  -- lower right
+                  then up - 1
+                  else up
             mfrac' = modifymfrac (mfrac ++ repeat 1) delt pos
         in l { threeColSlaves = take total mfrac'}
       modifymfrac [] _ _ = []