make right-click in the space between the definitions and

interactions window pop up a menu to let you change from
vertical to horizontal orientation

collects/drracket/private/unit.rkt

@@ -1100,7 +1100,8 @@ module browser threading seems wrong.
   (define dragable/def-int-mixin
     (mixin (panel:dragable<%>) ()
       (init-field unit-frame)
-      (inherit get-percentages)
+      (inherit get-percentages popup-menu 
+               set-orientation get-vertical?)
       (define/augment (after-percentage-change)
         (let ([percentages (get-percentages)])
           (when (and (= 1
@@ -1109,6 +1110,21 @@ module browser threading seems wrong.
                      (= 2 (length percentages)))
             (preferences:set 'drracket:unit-window-size-percentage (car percentages))))
         (inner (void) after-percentage-change))
+      (define/override (right-click-in-gap evt before after)
+        (define menu (new popup-menu%))
+        (define vertical? (get-vertical?))
+        (new menu-item%
+             [parent menu]
+             [label 
+              ;; something seems to be wrong with the get-vertical? method...
+              (if vertical?
+                  (string-constant change-to-vertical-alignment)
+                  (string-constant change-to-horizontal-alignment))]
+             [callback
+              (λ (a b) 
+                (preferences:set 'drracket:defs/ints-horizontal vertical?)
+                (set-orientation vertical?))])
+        (popup-menu menu (send evt get-x) (send evt get-y)))
       (super-new)))
   
   (define vertical-dragable/def-int% (dragable/def-int-mixin panel:vertical-dragable%))
@@ -4112,8 +4128,6 @@ module browser threading seems wrong.
                                        vertical-dragable/def-int%)
                                    (unit-frame this)
                                    (parent panel-with-tabs))]
-      [define orientation-callback (λ (p v) (send resizable-panel set-orientation v))]
-      (preferences:add-callback 'drracket:defs/ints-horizontal orientation-callback #t)
       
       [define definitions-canvas #f]
       (initialize-definitions-canvas)

collects/framework/private/panel.rkt

@@ -171,7 +171,9 @@
       set-percentages
       get-percentages
       get-vertical?
-      get-default-percentages))
+      get-default-percentages
+      right-click-in-gap
+      set-orientation))
   
   (define vertical-dragable<%>
     (interface (dragable<%>)))
@@ -184,7 +186,6 @@
       (init parent)
 
       (init-field vertical?)
-
       (define/public-final (get-vertical?) vertical?)
       (define/public-final (set-orientation h?) 
         (define v? (not h?))
@@ -205,6 +206,8 @@
             (icon:get-up/down-cursor)
             (icon:get-left/right-cursor)))
       
+      (define/public (right-click-in-gap evt before after) (void))
+      
       (inherit get-client-size container-flow-modified)
       
       (init-field [bar-thickness 5])
@@ -285,6 +288,8 @@
                                  (and (send c ok?)
                                       c))))
               (cond
+                [(and gap (send evt button-down? 'right))
+                 (right-click-in-gap evt (gap-before gap) (gap-after gap))]
                 [(and gap (send evt button-down? 'left))
                  (set! resizing-dim (event-get-dim evt))
                  (set! resizing-gap gap)]

collects/scribblings/framework/panel.scrbl

@@ -81,6 +81,16 @@
      The numbers in the result list must sum to @racket[1].
   }
 
+  @defmethod[(right-click-in-gap [evt (is-a?/c mouse-event%)]
+                                 [before (is-a?/c subarea<%>)]
+                                 [after (is-a?/c subarea<%>)])
+             void?]{
+    This method is called when the user right-clicks in the space
+    between two children. It receives the mouse event and the
+    child before and after the gap where the user clicked.
+  }
+
+                                                    
   @defmethod*[(((set-percentages (new-percentages (listof number?))) void?))]{
     Call this method to set the percentages that each window takes up of the
     panel.

collects/string-constants/private/english-string-constants.rkt

@@ -807,6 +807,11 @@ please adhere to these guidelines:
   "Would you like to create a new window, or clear out the current one?")
  (clear-current "Clear Current")
  (new-window "New Window")
+  
+  ;; popup menu when right-clicking in the gap between
+  ;; the definitions and interactions window
+  (change-to-vertical-alignment "Change to vertical")
+  (change-to-horizontal-alignment "Change to horizontal")
 
  ;;; exiting and quitting ``are you sure'' dialog
  ;;; exit is used on windows, quit on macos, in English. Other