diff --git a/collects/framework/private/panel.rkt b/collects/framework/private/panel.rkt
index 71bd9a8de1..a75ecb9191 100644
--- a/collects/framework/private/panel.rkt
+++ b/collects/framework/private/panel.rkt
@@ -1,6 +1,7 @@
 #lang racket/unit
 
-  (require mzlib/class
+  (require racket/class
+           racket/list
            "sig.rkt"
            mred/mred-sig)
 
@@ -169,7 +170,8 @@
       after-percentage-change
       set-percentages
       get-percentages
-      get-vertical?))
+      get-vertical?
+      get-default-percentages))
   
   (define vertical-dragable<%>
     (interface (dragable<%>)))
@@ -244,12 +246,23 @@
           (unless (= len-children (length percentages))
             (cond
               [(zero? len-children)
-               '()]
+               (set! percentages '())]
               [else
-               (let ([rat (/ 1 len-children)])
-                 (set! percentages (build-list len-children (λ (i) (make-percentage rat)))))])
+               (set! percentages (map make-percentage (get-default-percentages len-children)))])
             (after-percentage-change))))
       
+      (define/pubment (get-default-percentages i) 
+        (define res (inner (if (zero? i) '() (make-list i (/ i)))
+                           get-default-percentages i))
+        (unless (and (list? res)
+                     (andmap (λ (x) (and (real? x) (<= 0 x 1))) res)
+                     (= 1 (apply + res))
+                     (= (length res) i))
+          (error 'get-default-percentages 
+                 "expected inner call to return a list of real numbers that sum to 1 and has length ~a"
+                 i))
+        res)
+      
       (define/override (after-new-child child)
         (update-percentages))
       
diff --git a/collects/gui-debugger/debug-tool.rkt b/collects/gui-debugger/debug-tool.rkt
index 0ca426e48e..e655cc7504 100644
--- a/collects/gui-debugger/debug-tool.rkt
+++ b/collects/gui-debugger/debug-tool.rkt
@@ -24,6 +24,10 @@
   
   (define-local-member-name debug-callback)
   
+  (preferences:set-default 'plt:debug-tool:stack/variable-area
+                           9/10 
+                           (λ (x) (and (real? x) (<= 0 x 1))))
+  
   (define tool@
     (unit 
       (import drscheme:tool^)
@@ -1215,8 +1219,21 @@
           (define mouse-over-frame #f)
           (define/override (get-definitions/interactions-panel-parent)
             (set! debug-grandparent-panel
-                  (make-object horizontal-panel%
-                    (super get-definitions/interactions-panel-parent)))
+                  (new (class panel:horizontal-dragable%
+                         (inherit get-percentages)
+                         (define/augment (get-default-percentages i)
+                           (cond
+                             [(= i 2) 
+                              (define p (preferences:get 'plt:debug-tool:stack/variable-area))
+                              (list p (- 1 p))]
+                             [else (build-list i (λ (x) (/ i)))]))
+                         (define/augment (after-percentage-change)
+                           (define ps (get-percentages))
+                           (when (= (length ps) 2)
+                             (preferences:set 'plt:debug-tool:stack/variable-area (car ps)))
+                           (inner (void) after-percentage-change))
+                         (super-new))
+                       [parent (super get-definitions/interactions-panel-parent)]))
             (set! stack-view-panel
                   (new panel:vertical-dragable%
                        [parent debug-grandparent-panel]
diff --git a/collects/scribblings/framework/panel.scrbl b/collects/scribblings/framework/panel.scrbl
index f48b41435c..7215d289e1 100644
--- a/collects/scribblings/framework/panel.scrbl
+++ b/collects/scribblings/framework/panel.scrbl
@@ -45,7 +45,12 @@
 }
 
 @defmixin[panel:single-window-mixin (panel:single<%> window<%>) (panel:single-window<%>)]{
-  @defmethod*[#:mode override (((container-size (info (listof (list/c exact-integer? exact-integer? boolean? boolean?)))) (values exact-integer? exact-integer?)))]{
+  @defmethod*[#:mode override 
+                     (((container-size (info (listof (list/c exact-integer? 
+                                                             exact-integer? 
+                                                             boolean? 
+                                                             boolean?))))
+                       (values exact-integer? exact-integer?)))]{
     Factors the border width into the size calculation.
   }
 }
@@ -66,6 +71,15 @@
     Use @method[panel:dragable<%> get-percentages] to find the current
     percentages.
   }
+  
+  @defmethod[(get-default-percentages [subwindow-count exact-positive-integer?])
+             (listof (and/c real? (between/c 0 1)))]{
+     Called when the number of children in the panel changes;
+     the result is used as the initial percentages for each of the new
+     windows. 
+     
+     The numbers in the result list must sum to @racket[1].
+  }
 
   @defmethod*[(((set-percentages (new-percentages (listof number?))) void?))]{
     Call this method to set the percentages that each window takes up of the