Bug 532732. When rendering a background-attachment:fixed background to the screen, limit the background painting area to the viewport, since this is the only area where it will actually be visible and in many cases this means we don't need to think about tiling it. r=dbaron

layout/base/nsCSSRendering.cpp

@@ -2367,11 +2367,15 @@ PaintBackgroundLayer(nsPresContext* aPresContext,
     }
   }
 
+  // For background-attachment:fixed backgrounds, we'll limit the area
+  // where the background can be drawn to the viewport.
+  nsRect bgClipRect = aBGClipRect;
+
   // Compute the anchor point.
   //
   // relative to aBorderArea.TopLeft() (which is where the top-left
   // of aForFrame's border-box will be rendered)
-  nsPoint imageTopLeft, anchor, offset;
+  nsPoint imageTopLeft, anchor;
   if (NS_STYLE_BG_ATTACHMENT_FIXED == aLayer.mAttachment) {
     // If it's a fixed background attachment, then the image is placed
     // relative to the viewport, which is the area of the root frame
@@ -2389,8 +2393,9 @@ PaintBackgroundLayer(nsPresContext* aPresContext,
       // else this is an embedded shell and its root frame is what we want
     }
 
-    // Set the background positioning area to the viewport's area.
-    bgPositioningArea.SetRect(nsPoint(0, 0), topFrame->GetSize());
+    // Set the background positioning area to the viewport's area
+    // (relative to aForFrame)
+    bgPositioningArea = nsRect(-aForFrame->GetOffsetTo(topFrame), topFrame->GetSize());
 
     if (!pageContentFrame) {
       // Subtract the size of scrollbars.
@@ -2402,9 +2407,16 @@ PaintBackgroundLayer(nsPresContext* aPresContext,
       }
     }
 
-    offset = bgPositioningArea.TopLeft() - aForFrame->GetOffsetTo(topFrame);
-  } else {
-    offset = bgPositioningArea.TopLeft();
+    if (aRenderingContext.ThebesContext()->GetFlags() &
+        gfxContext::FLAG_DESTINED_FOR_SCREEN) {
+      // Clip background-attachment:fixed backgrounds to the viewport, if we're
+      // painting to the screen. This avoids triggering tiling in common cases,
+      // without affecting output since drawing is always clipped to the viewport
+      // when we draw to the screen. (But it's not a pure optimization since it
+      // can affect the values of pixels at the edge of the viewport ---
+      // whether they're sampled from a putative "next tile" or not.)
+      bgClipRect.IntersectRect(bgClipRect, bgPositioningArea + aBorderArea.TopLeft());
+    }
   }
 
   nsSize imageSize = imageRenderer.ComputeSize(bgPositioningArea.Size());
@@ -2458,8 +2470,8 @@ PaintBackgroundLayer(nsPresContext* aPresContext,
   // determined.
   ComputeBackgroundAnchorPoint(aLayer, bgPositioningArea.Size(), imageSize,
                                &imageTopLeft, &anchor);
-  imageTopLeft += offset;
-  anchor += offset;
+  imageTopLeft += bgPositioningArea.TopLeft();
+  anchor += bgPositioningArea.TopLeft();
 
   nsRect destArea(imageTopLeft + aBorderArea.TopLeft(), imageSize);
   nsRect fillArea = destArea;
@@ -2467,14 +2479,14 @@ PaintBackgroundLayer(nsPresContext* aPresContext,
   PR_STATIC_ASSERT(NS_STYLE_BG_REPEAT_XY ==
                    (NS_STYLE_BG_REPEAT_X | NS_STYLE_BG_REPEAT_Y));
   if (repeat & NS_STYLE_BG_REPEAT_X) {
-    fillArea.x = aBGClipRect.x;
-    fillArea.width = aBGClipRect.width;
+    fillArea.x = bgClipRect.x;
+    fillArea.width = bgClipRect.width;
   }
   if (repeat & NS_STYLE_BG_REPEAT_Y) {
-    fillArea.y = aBGClipRect.y;
-    fillArea.height = aBGClipRect.height;
+    fillArea.y = bgClipRect.y;
+    fillArea.height = bgClipRect.height;
   }
-  fillArea.IntersectRect(fillArea, aBGClipRect);
+  fillArea.IntersectRect(fillArea, bgClipRect);
 
   imageRenderer.Draw(aPresContext, aRenderingContext, destArea, fillArea,
                      anchor + aBorderArea.TopLeft(), aDirtyRect);