--- orig/core.lisp
+++ mod/core.lisp
@@ -633,15 +633,21 @@
 			      (let ((f (frame-data screen n)))
 				(frame-y f)))))
 
+(defun tree-right (screen tree)
+  (tree-accum-fn tree #'max (lambda (n)
+			      (let ((f (frame-data screen n)))
+				(+ (frame-x f) (frame-width f))))))
+
+(defun tree-bottom (screen tree)
+  (tree-accum-fn tree #'max (lambda (n)
+			      (let ((f (frame-data screen n)))
+				(+ (frame-y f) (frame-height f))))))
+
 (defun tree-width (screen tree)
-  (tree-accum-fn tree #'+ (lambda (n)
-			    (let ((f (frame-data screen n)))
-			      (frame-width f)))))
+  (- (tree-right screen tree) (tree-x screen tree)))
 
 (defun tree-height (screen tree)
-  (tree-accum-fn tree #'+ (lambda (n)
-			    (let ((f (frame-data screen n)))
-			      (frame-height f)))))
+  (- (tree-bottom screen tree) (tree-y screen tree)))
 
 (defun tree-row-split (screen tree)
   "Return t if the children of tree are stacked vertically"