Handle failed attempts to split a side window (Bug#18304).

* window.el (window--side-window-p): New function.
(split-window, window-splittable-p): Use window--side-window-p to
determine whether WINDOW can be split (Bug#18304).
* calendar/calendar.el (calendar-basic-setup): Fix one call of
`window-splittable-p' and add another (Bug#18304).

lisp/ChangeLog

@@ -1,3 +1,11 @@
+2014-08-21  Martin Rudalics  <rudalics@gmx.at>
+
+	* window.el (window--side-window-p): New function.
+	(split-window, window-splittable-p): Use window--side-window-p to
+	determine whether WINDOW can be split (Bug#18304).
+	* calendar/calendar.el (calendar-basic-setup): Fix one call of
+	`window-splittable-p' and add another (Bug#18304).
+
 2014-08-20  Sam Steingold  <sds@gnu.org>
 
 	* progmodes/python.el (python-new-pythonpath): Extract from

lisp/calendar/calendar.el

@@ -1423,10 +1423,16 @@ display the generated calendar."
         ;; the right thing in that case.
         ;;
         ;; Is this a wide frame?  If so, split it horizontally.
-        (if (window-splittable-p t) (split-window-right))
+
+	;; The following doesn't sound useful: If we split horizontally
+	;; here, the subsequent `pop-to-buffer' will likely split again
+	;; horizontally and we end up with three side-by-side windows.
+        (when (window-splittable-p (selected-window) t)
+	  (split-window-right))
         (pop-to-buffer calendar-buffer)
         ;; Has the window already been split vertically?
         (when (and (not (window-dedicated-p))
+		   (window-splittable-p (selected-window))
                    (window-full-height-p))
           (let ((win (split-window-below)))
             ;; In the upper window, show whatever was visible before.

lisp/window.el

@@ -743,6 +743,15 @@ number of slots on that side."
      (integer :tag "Number" :value 3 :size 5)))
   :group 'windows)
 
+(defun window--side-window-p (window)
+  "Return non-nil if WINDOW is a side window or the parent of one."
+  (or (window-parameter window 'window-side)
+      (and (window-child window)
+	   (or (window-parameter
+		(window-child window) 'window-side)
+	       (window-parameter
+		(window-last-child window) 'window-side)))))
+
 (defun window--major-non-side-window (&optional frame)
   "Return the major non-side window of frame FRAME.
 The optional argument FRAME must be a live frame and defaults to
@@ -4378,12 +4387,7 @@ frame.  The selected window is not changed by this function."
        ;; side window, throw an error unless `window-combination-resize'
        ;; equals 'side.
        ((and (not (eq window-combination-resize 'side))
-	     (or (window-parameter window 'window-side)
-		 (and (window-child window)
-		      (or (window-parameter
-			   (window-child window) 'window-side)
-			  (window-parameter
-			   (window-last-child window) 'window-side)))))
+	     (window--side-window-p window))
 	(error "Cannot split side window or parent of side window"))
        ;; If `window-combination-resize' is 'side and window has a side
        ;; window sibling, bind `window-combination-limit' to t.
@@ -5798,7 +5802,7 @@ hold:
   wide as `split-width-threshold'.
 - When WINDOW is split evenly, the emanating windows are at least
   `window-min-width' or two (whichever is larger) columns wide."
-  (when (window-live-p window)
+  (when (and (window-live-p window) (not (window--side-window-p window)))
     (with-current-buffer (window-buffer window)
       (if horizontal
 	  ;; A window can be split horizontally when its width is not