fix for #163404. when the height of a tree shrinks, we should ensure that the selected row is still visible. this really comes into play in the mailnews thread pane.

when I show the message pane (after it has been collapsed), the selected message can go out of view.

if the size of the thread pane shrinks because I move the splitter or make the whole window smaller, it can happen as well.

Note Outlook Express does the same thing.

r=varga, sr=bryner.  thanks to martinl for the bug report.

layout/xul/base/src/tree/src/nsTreeBodyFrame.cpp

@@ -634,9 +634,20 @@ nsTreeBodyFrame::ReflowFinished(nsIPresShell* aPresShell, PRBool* aFlushFlag)
     PRInt32 rowCount;
     mView->GetRowCount(&rowCount);
     PRInt32 lastPageTopRow = PR_MAX(0, rowCount - mPageCount);
-    if (mTopRowIndex >= lastPageTopRow)
+    if (mTopRowIndex > lastPageTopRow)
       ScrollToRow(lastPageTopRow);
 
+    // make sure that the current selected item is still
+    // visible after the tree changes size.
+    nsCOMPtr<nsITreeSelection> sel;
+    mView->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      PRInt32 currentIndex;
+      sel->GetCurrentIndex(&currentIndex);
+      if (currentIndex != -1)
+        EnsureRowIsVisible(currentIndex);
+    }
+
     InvalidateScrollbar();
     CheckVerticalOverflow();
   }