+void wxGrid::HighlightBlock( int topRow, int leftCol, int bottomRow, int rightCol )
+{
+    int temp;
+    wxGridCellCoords updateTopLeft, updateBottomRight;
+
+    if ( m_selection )
+    {
+        if ( m_selection->GetSelectionMode() == wxGrid::wxGridSelectRows )
+        {
+            leftCol = 0;
+            rightCol = GetNumberCols() - 1;
+        }
+        else if ( m_selection->GetSelectionMode() == wxGrid::wxGridSelectColumns )
+        {
+            topRow = 0;
+            bottomRow = GetNumberRows() - 1;
+        }
+    }
+
+    if ( topRow > bottomRow )
+    {
+        temp = topRow;
+        topRow = bottomRow;
+        bottomRow = temp;
+    }
+
+    if ( leftCol > rightCol )
+    {
+        temp = leftCol;
+        leftCol = rightCol;
+        rightCol = temp;
+    }
+
+    updateTopLeft = wxGridCellCoords( topRow, leftCol );
+    updateBottomRight = wxGridCellCoords( bottomRow, rightCol );
+
+    // First the case that we selected a completely new area
+    if ( m_selectingTopLeft == wxGridNoCellCoords ||
+         m_selectingBottomRight == wxGridNoCellCoords )
+    {
+        wxRect rect;
+        rect = BlockToDeviceRect( wxGridCellCoords ( topRow, leftCol ),
+                                  wxGridCellCoords ( bottomRow, rightCol ) );
+        m_gridWin->Refresh( false, &rect );
+    }
+
+    // Now handle changing an existing selection area.
+    else if ( m_selectingTopLeft != updateTopLeft ||
+              m_selectingBottomRight != updateBottomRight )
+    {
+        // Compute two optimal update rectangles:
+        // Either one rectangle is a real subset of the
+        // other, or they are (almost) disjoint!
+        wxRect  rect[4];
+        bool    need_refresh[4];
+        need_refresh[0] =
+        need_refresh[1] =
+        need_refresh[2] =
+        need_refresh[3] = false;
+        int     i;
+
+        // Store intermediate values
+        wxCoord oldLeft = m_selectingTopLeft.GetCol();
+        wxCoord oldTop = m_selectingTopLeft.GetRow();
+        wxCoord oldRight = m_selectingBottomRight.GetCol();
+        wxCoord oldBottom = m_selectingBottomRight.GetRow();
+
+        // Determine the outer/inner coordinates.
+        if (oldLeft > leftCol)
+        {
+            temp = oldLeft;
+            oldLeft = leftCol;
+            leftCol = temp;
+        }
+        if (oldTop > topRow )
+        {
+            temp = oldTop;
+            oldTop = topRow;
+            topRow = temp;
+        }
+        if (oldRight < rightCol )
+        {
+            temp = oldRight;
+            oldRight = rightCol;
+            rightCol = temp;
+        }
+        if (oldBottom < bottomRow)
+        {
+            temp = oldBottom;
+            oldBottom = bottomRow;
+            bottomRow = temp;
+        }
+
+        // Now, either the stuff marked old is the outer
+        // rectangle or we don't have a situation where one
+        // is contained in the other.
+
+        if ( oldLeft < leftCol )
+        {
+            // Refresh the newly selected or deselected
+            // area to the left of the old or new selection.
+            need_refresh[0] = true;
+            rect[0] = BlockToDeviceRect(
+                wxGridCellCoords( oldTop,  oldLeft ),
+                wxGridCellCoords( oldBottom, leftCol - 1 ) );
+        }
+
+        if ( oldTop < topRow )
+        {
+            // Refresh the newly selected or deselected
+            // area above the old or new selection.
+            need_refresh[1] = true;
+            rect[1] = BlockToDeviceRect(
+                wxGridCellCoords( oldTop, leftCol ),
+                wxGridCellCoords( topRow - 1, rightCol ) );
+        }
+
+        if ( oldRight > rightCol )
+        {
+            // Refresh the newly selected or deselected
+            // area to the right of the old or new selection.
+            need_refresh[2] = true;
+            rect[2] = BlockToDeviceRect(
+                wxGridCellCoords( oldTop, rightCol + 1 ),
+                wxGridCellCoords( oldBottom, oldRight ) );
+        }
+
+        if ( oldBottom > bottomRow )
+        {
+            // Refresh the newly selected or deselected
+            // area below the old or new selection.
+            need_refresh[3] = true;
+            rect[3] = BlockToDeviceRect(
+                wxGridCellCoords( bottomRow + 1, leftCol ),
+                wxGridCellCoords( oldBottom, rightCol ) );
+        }
+
+        // various Refresh() calls
+        for (i = 0; i < 4; i++ )
+            if ( need_refresh[i] && rect[i] != wxGridNoCellRect )
+                m_gridWin->Refresh( false, &(rect[i]) );
+    }
+
+    // change selection
+    m_selectingTopLeft = updateTopLeft;
+    m_selectingBottomRight = updateBottomRight;
+}