Bug 1324591 - Add APZSampler::GetCompositionBounds to get the composition bounds for a given pair of LayersId and scrollId. r=botond

Depends on D75730

Differential Revision: https://phabricator.services.mozilla.com/D81481

gfx/layers/apz/public/APZSampler.h

@@ -98,6 +98,14 @@ class APZSampler {
   void MarkAsyncTransformAppliedToContent(const LayerMetricsWrapper& aLayer);
   bool HasUnusedAsyncTransform(const LayerMetricsWrapper& aLayer);
 
+  /**
+   * Returns the composition bounds of the APZC correspoinding to the pair of
+   * |aLayersId| and |aScrollId|.
+   */
+  ParentLayerRect GetCompositionBounds(
+      const LayersId& aLayersId,
+      const ScrollableLayerGuid::ViewID& aScrollId) const;
+
   ScrollableLayerGuid GetGuid(const LayerMetricsWrapper& aLayer);
 
   ScreenMargin GetGeckoFixedLayerMargins() const;

gfx/layers/apz/src/APZSampler.cpp

@@ -212,6 +212,28 @@ ScreenMargin APZSampler::GetGeckoFixedLayerMargins() const {
   return mApz->GetGeckoFixedLayerMargins();
 }
 
+ParentLayerRect APZSampler::GetCompositionBounds(
+    const LayersId& aLayersId,
+    const ScrollableLayerGuid::ViewID& aScrollId) const {
+  // This function can get called on the compositor in case of non WebRender
+  // get called on the sampler thread in case of WebRender.
+  AssertOnSamplerThread();
+
+  RefPtr<AsyncPanZoomController> apzc =
+      mApz->GetTargetAPZC(aLayersId, aScrollId);
+  if (!apzc) {
+    // On WebRender it's possible that this function can get called even after
+    // the target APZC has been already destroyed because destroying the
+    // animation which triggers this function call is basically processed later
+    // than the APZC one, i.e. queue mCompositorAnimationsToDelete in
+    // WebRenderBridgeParent and then remove them in
+    // WebRenderBridgeParent::RemoveEpochDataPriorTo.
+    return ParentLayerRect();
+  }
+
+  return apzc->GetCompositionBounds();
+}
+
 void APZSampler::AssertOnSamplerThread() const {
   if (APZThreadUtils::GetThreadAssertionsEnabled()) {
     MOZ_ASSERT(IsSamplerThread());