--- a/gfx/layers/opengl/ReusableTileStoreOGL.cpp
+++ b/gfx/layers/opengl/ReusableTileStoreOGL.cpp
@@ -213,30 +213,30 @@ ReusableTileStoreOGL::DrawTiles(TiledThe
   // rendered.
   gfxRect contentBounds, displayPort;
   ContainerLayer* scrollableLayer = nullptr;
   for (ContainerLayer* parent = aLayer->GetParent(); parent; parent = parent->GetParent()) {
       const FrameMetrics& parentMetrics = parent->GetFrameMetrics();
       if (parentMetrics.IsScrollable())
         scrollableLayer = parent;
       if (!parentMetrics.mDisplayPort.IsEmpty() && scrollableLayer) {
-          // Get the display-port bounds
+          // Get the display-port bounds in screen-space.
           displayPort = gfxRect(parentMetrics.mDisplayPort.x,
                                 parentMetrics.mDisplayPort.y,
                                 parentMetrics.mDisplayPort.width,
                                 parentMetrics.mDisplayPort.height);
 
-          // Check the scale transform applied to the root layer to determine
+          // Calculate the scale transform applied to the root layer to determine
           // the content resolution.
           Layer* rootLayer = aLayer->Manager()->GetRoot();
           const gfx3DMatrix& rootTransform = rootLayer->GetTransform();
           float scaleX = rootTransform.GetXScale();
           float scaleY = rootTransform.GetYScale();
 
-          // Get the content document bounds
+          // Get the content document bounds, in screen-space.
           const FrameMetrics& metrics = scrollableLayer->GetFrameMetrics();
           const nsIntSize& contentSize = metrics.mContentRect.Size();
           gfx::Point scrollOffset =
             gfx::Point((metrics.mScrollOffset.x * metrics.LayersPixelsPerCSSPixel().width) / scaleX,
                        (metrics.mScrollOffset.y * metrics.LayersPixelsPerCSSPixel().height) / scaleY);
           const nsIntPoint& contentOrigin = metrics.mContentRect.TopLeft() -
             nsIntPoint(NS_lround(scrollOffset.x), NS_lround(scrollOffset.y));
           gfxRect contentRect = gfxRect(contentOrigin.x, contentOrigin.y,
@@ -264,25 +264,27 @@ ReusableTileStoreOGL::DrawTiles(TiledThe
     if (aResolution != tile->mResolution)
       transformedValidRegion.ScaleRoundOut(1.0f/scaleFactor.width,
                                            1.0f/scaleFactor.height);
     nsIntRegion tileRegion;
     tileRegion.Sub(tile->mTileRegion, transformedValidRegion);
 
     // Subtract the display-port from the tile region.
     if (!displayPort.IsEmpty()) {
+      // Transform the display-port from screen space to layer space.
       gfxRect transformedRenderBounds = transform.Inverse().TransformBounds(displayPort);
       tileRegion.Sub(tileRegion, nsIntRect(transformedRenderBounds.x,
                                            transformedRenderBounds.y,
                                            transformedRenderBounds.width,
                                            transformedRenderBounds.height));
     }
 
     // Intersect the tile region with the content area.
     if (!contentBounds.IsEmpty()) {
+      // Transform the content bounds from screen space to layer space.
       gfxRect transformedRenderBounds = transform.Inverse().TransformBounds(contentBounds);
       tileRegion.And(tileRegion, nsIntRect(transformedRenderBounds.x,
                                            transformedRenderBounds.y,
                                            transformedRenderBounds.width,
                                            transformedRenderBounds.height));
     }
 
     // If the tile region is empty, skip drawing.