Bug 1273250 - Factor out a helper function to calculate the viewport rect. r=mstange

MozReview-Commit-ID: ACJayEWplCY

--HG--
extra : rebase_source : 22ef189053bd80e3f8faaa10fb239031f8010587
extra : histedit_source : 510dbd0887b2cecc5366d0be1cb380ec7efb3d0d

layout/base/nsDisplayList.cpp

@@ -2352,6 +2352,20 @@ RegisterThemeGeometry(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
   }
 }
 
+// Return the bounds of the viewport relative to |aFrame|'s reference frame.
+// Returns Nothing() if transforming into |aFrame|'s coordinate space fails.
+static Maybe<nsRect>
+GetViewportRectRelativeToReferenceFrame(nsDisplayListBuilder* aBuilder,
+                                        nsIFrame* aFrame)
+{
+  nsIFrame* rootFrame = aFrame->PresContext()->PresShell()->GetRootFrame();
+  nsRect rootRect = rootFrame->GetRectRelativeToSelf();
+  if (nsLayoutUtils::TransformRect(rootFrame, aFrame, rootRect) == nsLayoutUtils::TRANSFORM_SUCCEEDED) {
+    return Some(rootRect + aBuilder->ToReferenceFrame(aFrame));
+  }
+  return Nothing();
+}
+
 nsDisplayBackgroundImage::nsDisplayBackgroundImage(nsDisplayListBuilder* aBuilder,
                                                    nsIFrame* aFrame,
                                                    uint32_t aLayer,
@@ -3049,10 +3063,8 @@ nsDisplayBackgroundImage::GetBoundsInternal(nsDisplayListBuilder* aBuilder) {
     // If this is a background-attachment:fixed image, and APZ is enabled,
     // async scrolling could reveal additional areas of the image, so don't
     // clip it beyond clipping to the document's viewport.
-    nsIFrame* rootFrame = presContext->PresShell()->GetRootFrame();
-    nsRect rootRect = rootFrame->GetRectRelativeToSelf();
-    if (nsLayoutUtils::TransformRect(rootFrame, mFrame, rootRect) == nsLayoutUtils::TRANSFORM_SUCCEEDED) {
-      clipRect = clipRect.Union(rootRect + aBuilder->ToReferenceFrame(mFrame));
+    if (Maybe<nsRect> viewportRect = GetViewportRectRelativeToReferenceFrame(aBuilder, mFrame)) {
+      clipRect = clipRect.Union(*viewportRect);
     }
   }
   const nsStyleImageLayers::Layer& layer = mBackgroundStyle->mImage.mLayers[mLayer];