Bug 1583380. Collect fixed position info on APZCTreeManager and use that to provide transforms to webrender via SampleForWebRender. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D54403

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

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -152,6 +152,11 @@ struct APZCTreeManager::TreeBuildingState {
   // is cleared back to Nothing(). Note that this is only used in the WebRender
   // codepath.
   Maybe<uint64_t> mZoomAnimationId;
+
+  // This is populated with all the HitTestingTreeNodes that have a fixed
+  // position animation id (which indicates that they need to be sampled for
+  // WebRender on the sampler thread).
+  std::vector<HitTestingTreeNode*> mFixedPositionNodesWithAnimationId;
 };
 
 class APZCTreeManager::CheckerboardFlushObserver : public nsIObserver {
@@ -473,6 +478,10 @@ APZCTreeManager::UpdateHitTestingTreeImpl(const ScrollNode& aRoot,
           if (node->IsScrollThumbNode() && node->GetScrollbarAnimationId()) {
             state.mScrollThumbs.push_back(node);
           }
+          // GetFixedPositionAnimationId is only set when webrender is enabled.
+          if (node->GetFixedPositionAnimationId().isSome()) {
+            state.mFixedPositionNodesWithAnimationId.push_back(node);
+          }
           if (apzc && node->IsPrimaryHolder()) {
             state.mScrollTargets[apzc->GetGuid()] = node;
           }
@@ -618,6 +627,17 @@ APZCTreeManager::UpdateHitTestingTreeImpl(const ScrollNode& aRoot,
           thumb->GetScrollbarData(), targetGuid, target->GetTransform(),
           target->IsAncestorOf(thumb));
     }
+
+    mFixedPositionInfo.clear();
+    // For non-webrender, state.mFixedPositionNodesWithAnimationId will be empty
+    // so this will be a no-op.
+    for (HitTestingTreeNode* fixedPos :
+         state.mFixedPositionNodesWithAnimationId) {
+      MOZ_ASSERT(fixedPos->GetFixedPositionAnimationId().isSome());
+      mFixedPositionInfo.emplace_back(
+          fixedPos->GetFixedPositionAnimationId().value(),
+          fixedPos->GetFixedPosSides());
+    }
   }
 
   for (size_t i = 0; i < state.mNodesToDestroy.Length(); i++) {
@@ -740,6 +760,21 @@ void APZCTreeManager::SampleForWebRender(wr::TransactionWrapper& aTxn,
     transforms.AppendElement(
         wr::ToWrTransformProperty(info.mThumbAnimationId, transform));
   }
+
+  for (const FixedPositionInfo& info : mFixedPositionInfo) {
+    ScreenPoint translation =
+        AsyncCompositionManager::ComputeFixedMarginsOffset(
+            mCompositorFixedLayerMargins, info.mFixedPosSides,
+            mGeckoFixedLayerMargins);
+
+    LayerToParentLayerMatrix4x4 transform =
+        LayerToParentLayerMatrix4x4::Translation(ViewAs<ParentLayerPixel>(
+            translation, PixelCastJustification::ScreenIsParentLayerForRoot));
+
+    transforms.AppendElement(
+        wr::ToWrTransformProperty(info.mFixedPositionAnimationId, transform));
+  }
+
   aTxn.AppendTransformProperties(transforms);
 
   // Advance animations. It's important that this happens after

gfx/layers/apz/src/APZCTreeManager.h

@@ -791,7 +791,8 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    */
   bool mUsingAsyncZoomContainer;
 
-  /** A lock that protects mApzcMap and mScrollThumbInfo. */
+  /** A lock that protects mApzcMap, mScrollThumbInfo, and mFixedPositionInfo.
+   */
   mutable mozilla::Mutex mMapLock;
   /**
    * A map for quick access to get APZC instances by guid, without having to
@@ -842,6 +843,32 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    */
   std::vector<ScrollThumbInfo> mScrollThumbInfo;
 
+  /**
+   * A helper structure to store all the information needed to compute the
+   * async transform for a fixed position element on the sampler thread.
+   */
+  struct FixedPositionInfo {
+    uint64_t mFixedPositionAnimationId;
+    SideBits mFixedPosSides;
+
+    FixedPositionInfo(const uint64_t& aFixedPositionAnimationId,
+                      const SideBits aFixedPosSides)
+        : mFixedPositionAnimationId(aFixedPositionAnimationId),
+          mFixedPosSides(aFixedPosSides) {}
+  };
+  /**
+   * If this APZCTreeManager is being used with WebRender, this vector gets
+   * populated during a layers update. It holds a package of information needed
+   * to compute and set the async transforms on fixed position content. This
+   * information is extracted from the HitTestingTreeNodes for the WebRender
+   * case because accessing the HitTestingTreeNodes requires holding the tree
+   * lock which we cannot do on the WR sampler thread. mFixedPositionInfo,
+   * however, can be accessed while just holding the mMapLock which is safe to
+   * do on the sampler thread. mMapLock must be acquired while accessing or
+   * modifying mFixedPositionInfo.
+   */
+  std::vector<FixedPositionInfo> mFixedPositionInfo;
+
   /* Holds the zoom constraints for scrollable layers, as determined by the
    * the main-thread gecko code. This can only be accessed on the updater
    * thread. */