Bug 1568227. Render blobs at visibleRect.x,y instead of 0,0. r=nical

Differential Revision: https://phabricator.services.mozilla.com/D45322

--HG--
extra : moz-landing-system : lando

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -2047,6 +2047,7 @@ static bool PaintByLayer(nsDisplayItem* aItem,
 
 static bool PaintItemByDrawTarget(nsDisplayItem* aItem, gfx::DrawTarget* aDT,
                                   const LayoutDevicePoint& aOffset,
+                                  const IntRect& visibleRect,
                                   nsDisplayListBuilder* aDisplayListBuilder,
                                   const RefPtr<BasicLayerManager>& aManager,
                                   const gfx::Size& aScale,
@@ -2055,7 +2056,7 @@ static bool PaintItemByDrawTarget(nsDisplayItem* aItem, gfx::DrawTarget* aDT,
 
   bool isInvalidated = false;
   // XXX Why is this ClearRect() needed?
-  aDT->ClearRect(Rect(aDT->GetRect()));
+  aDT->ClearRect(Rect(visibleRect));
   RefPtr<gfxContext> context = gfxContext::CreateOrNull(aDT);
   MOZ_ASSERT(context);
 
@@ -2100,15 +2101,14 @@ static bool PaintItemByDrawTarget(nsDisplayItem* aItem, gfx::DrawTarget* aDT,
     // which isn't very useful.
     if (aHighlight) {
       aDT->SetTransform(gfx::Matrix());
-      aDT->FillRect(Rect(aDT->GetRect()),
-                    gfx::ColorPattern(aHighlight.value()));
+      aDT->FillRect(Rect(visibleRect), gfx::ColorPattern(aHighlight.value()));
     }
     if (aItem->Frame()->PresContext()->GetPaintFlashing() && isInvalidated) {
       aDT->SetTransform(gfx::Matrix());
       float r = float(rand()) / float(RAND_MAX);
       float g = float(rand()) / float(RAND_MAX);
       float b = float(rand()) / float(RAND_MAX);
-      aDT->FillRect(Rect(aDT->GetRect()),
+      aDT->FillRect(Rect(visibleRect),
                     gfx::ColorPattern(gfx::Color(r, g, b, 0.5)));
     }
   }
@@ -2195,7 +2195,7 @@ WebRenderCommandBuilder::GenerateFallbackData(
   if (dtSize.IsEmpty()) {
     return nullptr;
   }
-
+  // Display item bounds should be unscaled
   aImageRect = dtRect / layerScale;
 
   nsDisplayItemGeometry* geometry = fallbackData->mGeometry;
@@ -2276,8 +2276,11 @@ WebRenderCommandBuilder::GenerateFallbackData(
         fallbackData->mBasicLayerManager =
             new BasicLayerManager(BasicLayerManager::BLM_INACTIVE);
       }
+      // aOffset is (0, 0) because blobs don't want to normalize their
+      // coordinates
       bool isInvalidated = PaintItemByDrawTarget(
-          aItem, dt, LayoutDevicePoint(0, 0), aDisplayListBuilder,
+          aItem, dt, LayoutDevicePoint(0, 0),
+          /*aVisibleRect: */ dtRect.ToUnknownRect(), aDisplayListBuilder,
           fallbackData->mBasicLayerManager, scale, highlight);
       if (!isInvalidated) {
         if (!aItem->GetBuildingRect().IsEqualInterior(
@@ -2346,8 +2349,12 @@ WebRenderCommandBuilder::GenerateFallbackData(
             fallbackData->mBasicLayerManager =
                 new BasicLayerManager(mManager->GetWidget());
           }
+          // aOffset is applied because this case is a "real" image and not a
+          // blob
           isInvalidated = PaintItemByDrawTarget(
-              aItem, dt, aImageRect.TopLeft(), aDisplayListBuilder,
+              aItem, dt,
+              /*aOffset: */ aImageRect.TopLeft(),
+              /*aVisibleRect: */ dt->GetRect(), aDisplayListBuilder,
               fallbackData->mBasicLayerManager, scale, highlight);
         }