Bug 1263083 - Use single-tile layer when less than half the tile size in either dimension; r=mstange

MozReview-Commit-ID: 8yYXVl36AHK

--HG--
extra : transplant_source : %81%F9%DB%1D%F2%B9%A2vY%5D%04m%A0%1A%B7%EB%92%224P

gfx/layers/client/ClientTiledPaintedLayer.cpp

@@ -425,9 +425,16 @@ ClientTiledPaintedLayer::RenderLayer()
   IntSize layerSize = mVisibleRegion.ToUnknownRegion().GetBounds().Size();
   IntSize tileSize(gfxPlatform::GetPlatform()->GetTileWidth(),
                    gfxPlatform::GetPlatform()->GetTileHeight());
+  bool isHalfTileWidthOrHeight = layerSize.width <= tileSize.width / 2 ||
+    layerSize.height <= tileSize.height / 2;
 
+  // Use single tile when layer is not scrollable, is smaller than one
+  // tile, or when more than half of the tiles' pixels in either
+  // dimension would be wasted.
   bool wantSingleTiledContentClient =
-      (mCreationHint == LayerManager::NONE || layerSize <= tileSize) &&
+      (mCreationHint == LayerManager::NONE ||
+       layerSize <= tileSize ||
+       isHalfTileWidthOrHeight) &&
       SingleTiledContentClient::ClientSupportsLayerSize(layerSize, ClientManager()) &&
       gfxPrefs::LayersSingleTileEnabled();