--- a/layout/xul/tree/nsTreeBodyFrame.cpp
+++ b/layout/xul/tree/nsTreeBodyFrame.cpp
@@ -6,16 +6,17 @@
 #include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/ContentEvents.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/MouseEvents.h"
 #include "mozilla/Likely.h"
 
+#include "nsAlgorithm.h"
 #include "nsCOMPtr.h"
 #include "nsPresContext.h"
 #include "nsNameSpaceManager.h"
 
 #include "nsTreeBodyFrame.h"
 #include "nsTreeSelection.h"
 #include "nsTreeImageListener.h"
 
@@ -4041,88 +4042,65 @@ nsTreeBodyFrame::ScrollToHorizontalPosit
   UpdateScrollbars(parts);
   return rv;
 }
 
 nsresult
 nsTreeBodyFrame::ScrollToRow(int32_t aRow)
 {
   ScrollParts parts = GetScrollParts();
-  nsresult rv = ScrollToRowInternal(parts, aRow);
-  NS_ENSURE_SUCCESS(rv, rv);
+  ScrollToRowInternal(parts, aRow);
   UpdateScrollbars(parts);
-  return rv;
+  return NS_OK;
 }
 
 nsresult nsTreeBodyFrame::ScrollToRowInternal(const ScrollParts& aParts, int32_t aRow)
 {
   ScrollInternal(aParts, aRow);
 
   return NS_OK;
 }
 
 nsresult
 nsTreeBodyFrame::ScrollByLines(int32_t aNumLines)
 {
-  if (!mView)
+  if (!mView) {
     return NS_OK;
-
+  }
   int32_t newIndex = mTopRowIndex + aNumLines;
-  if (newIndex < 0)
-    newIndex = 0;
-  else {
-    int32_t lastPageTopRow = mRowCount - mPageLength;
-    if (newIndex > lastPageTopRow)
-      newIndex = lastPageTopRow;
-  }
   ScrollToRow(newIndex);
-  
   return NS_OK;
 }
 
 nsresult
 nsTreeBodyFrame::ScrollByPages(int32_t aNumPages)
 {
-  if (!mView)
+  if (!mView) {
     return NS_OK;
-
+  }
   int32_t newIndex = mTopRowIndex + aNumPages * mPageLength;
-  if (newIndex < 0)
-    newIndex = 0;
-  else {
-    int32_t lastPageTopRow = mRowCount - mPageLength;
-    if (newIndex > lastPageTopRow)
-      newIndex = lastPageTopRow;
-  }
   ScrollToRow(newIndex);
-    
   return NS_OK;
 }
 
 nsresult
 nsTreeBodyFrame::ScrollInternal(const ScrollParts& aParts, int32_t aRow)
 {
-  if (!mView)
+  if (!mView) {
     return NS_OK;
-
-  int32_t delta = aRow - mTopRowIndex;
-
-  if (delta > 0) {
-    if (mTopRowIndex == (mRowCount - mPageLength + 1))
-      return NS_OK;
   }
-  else {
-    if (mTopRowIndex == 0)
-      return NS_OK;
+  int32_t maxTopRowIndex = std::max(0, mRowCount - mPageLength);
+  MOZ_ASSERT(mTopRowIndex == mozilla::clamped(mTopRowIndex, 0, maxTopRowIndex));
+
+  aRow = mozilla::clamped(aRow, 0, maxTopRowIndex);
+  if (aRow == mTopRowIndex) {
+    return NS_OK;
   }
-
-  mTopRowIndex += delta;
-
+  mTopRowIndex = aRow;
   Invalidate();
-
   PostScrollEvent();
   return NS_OK;
 }
 
 nsresult
 nsTreeBodyFrame::ScrollHorzInternal(const ScrollParts& aParts, int32_t aPosition)
 {
   if (!mView || !aParts.mColumnsScrollFrame || !aParts.mHScrollbar)