Bug 1360112 - Correct background image position when enabling webrender background image. r=mattwoodrow

layout/base/nsLayoutUtils.cpp

@@ -6733,6 +6733,16 @@ nsLayoutUtils::ComputeSizeForDrawingWithFallback(imgIContainer* aImage,
   return imageSize;
 }
 
+/* static */ nsPoint
+nsLayoutUtils::GetBackgroundFirstTilePos(const nsPoint& aDest,
+                                         const nsPoint& aFill,
+                                         const nsSize& aRepeatSize)
+{
+  return nsPoint(NSToIntFloor(float(aFill.x - aDest.x) / aRepeatSize.width) * aRepeatSize.width,
+                 NSToIntFloor(float(aFill.y - aDest.y) / aRepeatSize.height) * aRepeatSize.height) +
+         aDest;
+}
+
 /* static */ DrawResult
 nsLayoutUtils::DrawBackgroundImage(gfxContext&         aContext,
                                    nsIFrame*           aForFrame,
@@ -6763,9 +6773,7 @@ nsLayoutUtils::DrawBackgroundImage(gfxContext&         aContext,
                              aOpacity);
   }
 
-  nsPoint firstTilePos = aDest.TopLeft() +
+  nsPoint firstTilePos = GetBackgroundFirstTilePos(aDest.TopLeft(), aFill.TopLeft(), aRepeatSize);
-                         nsPoint(NSToIntFloor(float(aFill.x - aDest.x) / aRepeatSize.width) * aRepeatSize.width,
-                                 NSToIntFloor(float(aFill.y - aDest.y) / aRepeatSize.height) * aRepeatSize.height);
   for (int32_t i = firstTilePos.x; i < aFill.XMost(); i += aRepeatSize.width) {
     for (int32_t j = firstTilePos.y; j < aFill.YMost(); j += aRepeatSize.height) {
       nsRect dest(i, j, aDest.width, aDest.height);

layout/base/nsLayoutUtils.h

@@ -1605,6 +1605,10 @@ public:
                          nsStyleContext*     aStyleContext = nullptr,
                          DrawStringFlags     aFlags = DrawStringFlags::eDefault);
 
+  static nsPoint GetBackgroundFirstTilePos(const nsPoint& aDest,
+                                           const nsPoint& aFill,
+                                           const nsSize& aRepeatSize);
+
   /**
    * Supports only LTR or RTL. Bidi (mixed direction) is not supported.
    */

layout/painting/nsImageRenderer.cpp

@@ -625,11 +625,17 @@ nsImageRenderer::BuildWebRenderDisplayItems(nsPresContext*       aPresContext,
           aDest, appUnitsPerDevPixel);
       LayerRect dest = aLayer->RelativeToParent(destRect);
 
+      nsPoint firstTilePos = nsLayoutUtils::GetBackgroundFirstTilePos(aDest.TopLeft(),
+                                                                      aFill.TopLeft(),
+                                                                      aRepeatSize);
       LayoutDeviceRect fillRect = LayoutDeviceRect::FromAppUnits(
-          aFill, appUnitsPerDevPixel);
+          nsRect(firstTilePos.x, firstTilePos.y,
+                 aFill.XMost() - firstTilePos.x, aFill.YMost() - firstTilePos.y),
+          appUnitsPerDevPixel);
       LayerRect fill = aLayer->RelativeToParent(fillRect);
+      LayerRect clip = aLayer->RelativeToParent(
+                                 LayoutDeviceRect::FromAppUnits(aFill,appUnitsPerDevPixel));
 
-      LayerRect clip = fill;
       Size gapSize((aRepeatSize.width - aDest.width) / appUnitsPerDevPixel,
                    (aRepeatSize.height - aDest.height) / appUnitsPerDevPixel);
       aBuilder.PushImage(wr::ToWrRect(fill), aBuilder.BuildClipRegion(wr::ToWrRect(clip)),