Check in Michiel van Leeuwen's patch for bug 135076 that fixes the problem where you get scroll arrows in submenus when the submenus are initially opened so close to the bottom of the screen that their position is adjusted to make them fit on screen. r=bryner, sr=ben.

layout/xul/base/src/nsMenuPopupFrame.cpp

@@ -1042,8 +1042,11 @@ nsMenuPopupFrame::SyncViewWithFrame(nsIPresContext* aPresContext,
   if (!parentView)
     return NS_OK;
 
+  // Use containingView instead of parentView, to account for the scrollarrows
+  // that a parent menu might have.
+
   nsCOMPtr<nsIWidget> parentViewWidget;
-  GetWidgetForView ( parentView, *getter_AddRefs(parentViewWidget) );
+  GetWidgetForView ( containingView, *getter_AddRefs(parentViewWidget) );
   nsRect localParentWidgetRect(0,0,0,0), screenParentWidgetRect;
   parentViewWidget->WidgetToScreen ( localParentWidgetRect, screenParentWidgetRect );
   PRInt32 screenViewLocX = NSIntPixelsToTwips(screenParentWidgetRect.x,p2t) + (xpos - parentPos.x);
@@ -1063,6 +1066,25 @@ nsMenuPopupFrame::SyncViewWithFrame(nsIPresContext* aPresContext,
     // shouldn't do anything funky if it will already fit on the screen as is.
     //
 
+    ///////////////////////////////////////////////////////////////////////////////
+    //
+    //                +------------------------+          
+    //                |           /\           |
+    // parentPos -> - +------------------------+
+    //              | |                        |
+    //       offset | |                        |
+    //              | |                        |
+    //              | |                        | (screenViewLocX,screenViewLocY)
+    //              - |========================|+--------------
+    //                | parentRect           > ||
+    //                |========================||
+    //                |                        || Submenu 
+    //                +------------------------+|  ( = mRect )
+    //                |           \/           ||
+    //                +------------------------+
+
+
+
     // compute screen coordinates of parent frame so we can play with it. Make sure we put it
     // into twips as everything else is as well.
     nsRect screenParentFrameRect ( NSTwipsToIntPixels(offset.x,t2p), NSTwipsToIntPixels(offset.y,t2p),
@@ -1121,7 +1143,24 @@ nsMenuPopupFrame::SyncViewWithFrame(nsIPresContext* aPresContext,
       }
       else {
         // move it up to be on screen, but don't let it go off the screen at the top
+
+        //  |
+        //  |
+        //  |+----  screenViewLocY
+        //  ||
+        //  ||  Submenu ( = mRect )
+        // -+|
+        //   |
+        //   |
+        // - - - - - - - - - - screenBottomTwips (bottom of the screen)
+        //   |    \ 
+        //   |     }  moveDistY
+        //   |    /
+        //   +----  screenViewLocY + mRect.height
+        //
+
         if ( (screenViewLocY + mRect.height) > screenBottomTwips ) {
+          // XXX Bug 84121 comment 48 says the next line has to use screenHeightTwips, why not screenBottomTwips?
           PRInt32 moveDistY = (screenViewLocY + mRect.height) - screenHeightTwips;
           if ( screenViewLocY - moveDistY < screenTopTwips )
             moveDistY = screenViewLocY - screenTopTwips;          
@@ -1133,10 +1172,13 @@ nsMenuPopupFrame::SyncViewWithFrame(nsIPresContext* aPresContext,
       // Resize it to fit on the screen. By this point, we've given the popup as much
       // room as we can w/out covering the parent. If it still can't be as big
       // as it wants to be, well, it just has to suck up and deal. 
+      //
+      // ySpillage is calculated the same way as moveDistY above. see picture there.
+
       PRInt32 xSpillage = (screenViewLocX + mRect.width) - screenRightTwips;
       if ( xSpillage > 0 )
         mRect.width -= xSpillage;
-      PRInt32 ySpillage = (screenViewLocY + mRect.height + parentRect.height) - screenHeightTwips;
+      PRInt32 ySpillage = (screenViewLocY + mRect.height) - screenBottomTwips;
       if ( ySpillage > 0 )
         mRect.height -= ySpillage;