--- a/gfx/layers/basic/BasicTiledThebesLayer.cpp
+++ b/gfx/layers/basic/BasicTiledThebesLayer.cpp
@@ -90,20 +90,20 @@ BasicTiledLayerBuffer::PaintThebes(Basic
   // If this region is empty XMost() - 1 will give us a negative value.
   NS_ASSERTION(!aPaintRegion.GetBounds().IsEmpty(), "Empty paint region\n");
 
   bool useSinglePaintBuffer = UseSinglePaintBuffer();
   if (useSinglePaintBuffer) {
     // Check if the paint only spans a single tile. If that's
     // the case there's no point in using a single paint buffer.
     nsIntRect paintBounds = aPaintRegion.GetBounds();
-    useSinglePaintBuffer = RoundDownToTileEdge(paintBounds.x) !=
-                           RoundDownToTileEdge(paintBounds.XMost() - 1) ||
-                           RoundDownToTileEdge(paintBounds.y) !=
-                           RoundDownToTileEdge(paintBounds.YMost() - 1);
+    useSinglePaintBuffer = GetTileStart(paintBounds.x) !=
+                           GetTileStart(paintBounds.XMost() - 1) ||
+                           GetTileStart(paintBounds.y) !=
+                           GetTileStart(paintBounds.YMost() - 1);
   }
 
   if (useSinglePaintBuffer) {
     const nsIntRect bounds = aPaintRegion.GetBounds();
     {
       SAMPLE_LABEL("BasicTiledLayerBuffer", "PaintThebesSingleBufferAlloc");
       mSinglePaintBuffer = new gfxImageSurface(gfxIntSize(bounds.width, bounds.height), GetFormat(), !aLayer->CanUseOpaqueSurface());
       mSinglePaintBufferOffset = nsIntPoint(bounds.x, bounds.y);