fix xmonad/xmonad#87

switch focus when mouse is entering a workspace
but not moving into a window (w' == 0)

for magic number 0 in w' == 0
see discussion in https://github.com/xmonad/xmonad/pull/88#pullrequestreview-62489502

CHANGES.md

@@ -14,6 +14,10 @@
     - Focus did not follow when moving between workspaces (#87)
     - etc.
 
+  * Recover old behavior (in 0.12) when `focusFollowsMouse == True`:
+    the focus follows when the mouse enters another workspace
+    but not moving into any window.
+
 ## 0.13 (February 10, 2017)
 
 ### Breaking Changes

src/XMonad/Main.hs

@@ -368,7 +368,9 @@ handle e@(CrossingEvent {ev_window = w, ev_event_type = t})
         dpy <- asks display
         root <- asks theRoot
         (_, _, w', _, _, _, _, _) <- io $ queryPointer dpy root
-        when (w == w') (focus w)
+        -- when Xlib cannot find a child that contains the pointer,
+        -- it returns None(0)
+        when (w' == 0 || w == w') (focus w)
 
 -- left a window, check if we need to focus root
 handle e@(CrossingEvent {ev_event_type = t})