Bug 1552608 - Handle fixed layer margins in ComputeTransformForNode(). r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D48379

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

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -3271,6 +3271,14 @@ LayerToParentLayerMatrix4x4 APZCTreeManager::ComputeTransformForNode(
                 scrollTargetNode->IsAncestorOf(aNode), nullptr);
           });
     }
+  } else if (IsFixedToRootContent(aNode)) {
+    ParentLayerPoint translation = ViewAs<ParentLayerPixel>(
+        AsyncCompositionManager::ComputeFixedMarginsOffset(
+            mFixedLayerMargins, aNode->GetFixedPosSides()),
+        PixelCastJustification::ScreenIsParentLayerForRoot);
+    return aNode->GetTransform() *
+           CompleteAsyncTransform(
+               AsyncTransformComponentMatrix::Translation(translation));
   }
   // Otherwise, the node does not have an async transform.
   return aNode->GetTransform() * AsyncTransformMatrix();