fix for #16929

nuke the scrollbar whenever the total rows is less than the visible rows. This prevents the scrollbar from disappearing under you if you scroll up after many rows have been deleted (such as collapsing folders in mail)
r=hyatt

layout/xul/base/src/nsTreeRowGroupFrame.cpp

@@ -930,12 +930,38 @@ nsTreeRowGroupFrame::ReflowAfterRowLayout(nsIPresContext&       aPresContext,
                                            nsReflowReason       aReason)
 {
   nsresult rv = NS_OK;
+
+  mRowCount = 0;
+  ComputeTotalRowCount(mRowCount, mContent); // XXX This sucks! Needs to be cheap!
+  // Our page size is the # of rows instantiated.
+  PRInt32 pageRowCount;
+  GetRowCount(pageRowCount);
+
   if (mScrollbar) {
+    PRBool nukeScrollbar=PR_FALSE;
+    nsAutoString value;
+    
     nsCOMPtr<nsIContent> scrollbarContent;
     mScrollbar->GetContent(getter_AddRefs(scrollbarContent));
-    nsString value;
-    scrollbarContent->GetAttribute(kNameSpaceID_None, nsXULAtoms::curpos, value);
-    if (value == "0" && !mIsFull) {
+    
+    if (mRowCount <= pageRowCount) {
+      // first set the position to 0 so that all visible content
+      // scrolls into view
+      value.Append(0);
+      scrollbarContent->SetAttribute(kNameSpaceID_None,
+                                     nsXULAtoms::curpos,
+                                     value, PR_TRUE);
+      // now force nuking the scrollbar
+      // (otherwise it takes a bunch of reflows to actually make it go away)
+      nukeScrollbar=PR_TRUE;
+    }
+
+    else {
+      scrollbarContent->GetAttribute(kNameSpaceID_None,
+                                     nsXULAtoms::curpos, value);
+    }
+   
+    if (nukeScrollbar || (value == "0" && !mIsFull)) {
       // Nuke the scrollbar.
       mFrameConstructor->RemoveMappingsForFrameSubtree(&aPresContext, mScrollbar);
       mScrollbarList.DestroyFrames(aPresContext);
@@ -943,9 +969,6 @@ nsTreeRowGroupFrame::ReflowAfterRowLayout(nsIPresContext&       aPresContext,
     }
   }
 
-  mRowCount = 0;
-  ComputeTotalRowCount(mRowCount, mContent); // XXX This sucks! Needs to be cheap!
-
   if (mShouldHaveScrollbar && (mRowGroupHeight != NS_UNCONSTRAINEDSIZE) &&
       (mIsFull || mScrollbar)) {
     // Ensure the scrollbar has been created.
@@ -967,9 +990,6 @@ nsTreeRowGroupFrame::ReflowAfterRowLayout(nsIPresContext&       aPresContext,
       scrollbarContent->GetAttribute(kNameSpaceID_None, nsXULAtoms::curpos, maxpos);
     }
     else {
-      // Our page size is the # of rows instantiated.
-      PRInt32 pageRowCount;
-      GetRowCount(pageRowCount);
 
       if (pageRowCount < 2)
         pageRowCount = 2;
@@ -1847,4 +1867,3 @@ nsTreeRowGroupFrame :: Paint ( nsIPresContext& aPresContext, nsIRenderingContext
   return res;
   
 } // Paint
-