Bug 1546139 - Restore the call to AdjustFixedOrStickyLayer() for layers fixed to the RCD-RSF. r=kats

This call served two purposes: (1) scroll the fixed layer by the eVisual
transform, and (2) adjust it by the fixed margins. The first purpose is now
served by applying the eVisual transform to the async zoom container, but
we still need the call for the second purpose.

Differential Revision: https://phabricator.services.mozilla.com/D28735

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

gfx/layers/composite/AsyncCompositionManager.cpp

@@ -380,6 +380,32 @@ static bool AsyncTransformShouldBeUnapplied(
   return false;
 }
 
+/**
+ * Given a fixed-position layer, check if it's fixed with respect to the
+ * zoomed APZC.
+ */
+static bool IsFixedToZoomContainer(Layer* aFixedLayer) {
+  ScrollableLayerGuid::ViewID targetId =
+      aFixedLayer->GetFixedPositionScrollContainerId();
+  MOZ_ASSERT(targetId != ScrollableLayerGuid::NULL_SCROLL_ID);
+  LayerMetricsWrapper result(aFixedLayer, LayerMetricsWrapper::StartAt::BOTTOM);
+  while (result) {
+    if (Maybe<ScrollableLayerGuid::ViewID> zoomedScrollId =
+            result.IsAsyncZoomContainer()) {
+      return *zoomedScrollId == targetId;
+    }
+    // Don't ascend into another layer tree. Scroll IDs are not unique
+    // across layer trees, and in any case position:fixed doesn't reach
+    // across documents.
+    if (result.AsRefLayer() != nullptr) {
+      break;
+    }
+
+    result = result.GetParent();
+  }
+  return false;
+}
+
 // If |aLayer| is fixed or sticky, returns the scroll id of the scroll frame
 // that it's fixed or sticky to. Otherwise, returns Nothing().
 static Maybe<ScrollableLayerGuid::ViewID> IsFixedOrSticky(Layer* aLayer) {
@@ -1197,6 +1223,27 @@ bool AsyncCompositionManager::ApplyAsyncContentTransformToTree(
               // scroll metadata for zoomedScrollId appears in the layer tree.
             }
           }
+
+          // Layers fixed to the RCD-RSF no longer need
+          // AdjustFixedOrStickyLayer() to scroll them by the eVisual transform,
+          // as that's now applied to the async zoom container itself. However,
+          // we still need to adjust them by the fixed layer margins to
+          // account for dynamic toolbar transitions. This is also handled by
+          // AdjustFixedOrStickyLayer(), so we now call it with empty transforms
+          // to get it to perform just the fixed margins adjustment.
+          if (zoomedMetrics && layer->GetIsFixedPosition() &&
+              !layer->GetParent()->GetIsFixedPosition() &&
+              IsFixedToZoomContainer(layer)) {
+            LayerToParentLayerMatrix4x4 emptyTransform;
+            ScreenMargin marginsForFixedLayer;
+#ifdef MOZ_WIDGET_ANDROID
+            marginsForFixedLayer = GetFixedLayerMargins();
+#endif
+            AdjustFixedOrStickyLayer(zoomContainer, layer,
+                                     sampler->GetGuid(*zoomedMetrics).mScrollId,
+                                     emptyTransform, emptyTransform,
+                                     marginsForFixedLayer, clipPartsCache);
+          }
         }
 
         bool clipChanged = (hasAsyncTransform || clipDeferredFromChildren ||