Bug 1053992 - Add a red square in the top-right corner of FPS display for unused APZ transforms (i.e. when content is sync-scrolling). r=BenWa

gfx/layers/composite/ContainerLayerComposite.cpp

@@ -424,6 +424,24 @@ ContainerRender(ContainerT* aContainer,
     RenderLayers(aContainer, aManager, RenderTargetPixel::FromUntyped(aClipRect));
   }
   aContainer->mPrepared = nullptr;
+
+  // If it is a scrollable container layer with no child layers, and one of the APZCs
+  // attached to it has a nonempty async transform, then that transform is not applied
+  // to any visible content. Display a warning box (conditioned on the FPS display being
+  // enabled).
+  if (gfxPrefs::LayersDrawFPS() && aContainer->IsScrollInfoLayer()) {
+    // Since aContainer doesn't have any children we can just iterate from the top metrics
+    // on it down to the bottom using GetFirstChild and not worry about walking onto another
+    // underlying layer.
+    for (LayerMetricsWrapper i(aContainer); i; i = i.GetFirstChild()) {
+      if (AsyncPanZoomController* apzc = i.GetApzc()) {
+        if (!Matrix4x4(apzc->GetCurrentAsyncTransform()).IsIdentity()) {
+          aManager->UnusedApzTransformWarning();
+          break;
+        }
+      }
+    }
+  }
 }
 
 ContainerLayerComposite::ContainerLayerComposite(LayerManagerComposite *aManager)

gfx/layers/composite/LayerManagerComposite.cpp

@@ -103,7 +103,9 @@ LayerManagerComposite::ClearCachedResources(Layer* aSubtree)
  * LayerManagerComposite
  */
 LayerManagerComposite::LayerManagerComposite(Compositor* aCompositor)
-: mCompositor(aCompositor)
+: mWarningLevel(0.0f)
+, mUnusedApzTransformWarning(false)
+, mCompositor(aCompositor)
 , mInTransaction(false)
 , mIsCompositorReady(false)
 , mDebugOverlayWantsNextFrame(false)
@@ -346,11 +348,11 @@ LayerManagerComposite::RenderDebugOverlay(const Rect& aBounds)
       mFPS = MakeUnique<FPSState>();
     }
 
+    float alpha = 1;
 #ifdef ANDROID
     // Draw a translation delay warning overlay
     int width;
     int border;
-    float alpha = 1;
     if ((now - mWarnTime).ToMilliseconds() < kVisualWarningDuration) {
       EffectChain effects;
 
@@ -385,6 +387,18 @@ LayerManagerComposite::RenderDebugOverlay(const Rect& aBounds)
 
     float fillRatio = mCompositor->GetFillRatio();
     mFPS->DrawFPS(now, drawFrameColorBars ? 10 : 0, 0, unsigned(fillRatio), mCompositor);
+
+    if (mUnusedApzTransformWarning) {
+      // If we have an unused APZ transform on this composite, draw a 20x20 red box
+      // in the top-right corner
+      EffectChain effects;
+      effects.mPrimaryEffect = new EffectSolidColor(gfx::Color(1, 0, 0, 1));
+      mCompositor->DrawQuad(gfx::Rect(aBounds.width - 20, 0, aBounds.width, 20),
+                            aBounds, effects, alpha, gfx::Matrix4x4());
+
+      mUnusedApzTransformWarning = false;
+      SetDebugOverlayWantsNextFrame(true);
+    }
   } else {
     mFPS = nullptr;
   }

gfx/layers/composite/LayerManagerComposite.h

@@ -257,6 +257,10 @@ public:
     }
   }
 
+  void UnusedApzTransformWarning() {
+    mUnusedApzTransformWarning = true;
+  }
+
 private:
   /** Region we're clipping our current drawing to. */
   nsIntRegion mClippingRegion;
@@ -297,6 +301,7 @@ private:
 
   float mWarningLevel;
   mozilla::TimeStamp mWarnTime;
+  bool mUnusedApzTransformWarning;
   RefPtr<Compositor> mCompositor;
   UniquePtr<LayerProperties> mClonedLayerTreeProperties;