Bug 958596 - Add a content description field to FrameMetrics and populate it if the apz.printtree pref is on. r=kats,tn

gfx/ipc/GfxMessageUtils.h

@@ -623,6 +623,7 @@ struct ParamTraits<mozilla::layers::FrameMetrics>
     WriteParam(aMsg, aParam.mDisableScrollingY);
     WriteParam(aMsg, aParam.mUpdateScrollOffset);
     WriteParam(aMsg, aParam.mScrollGeneration);
+    WriteParam(aMsg, aParam.mContentDescription);
   }
 
   static bool Read(const Message* aMsg, void** aIter, paramType* aResult)
@@ -645,7 +646,8 @@ struct ParamTraits<mozilla::layers::FrameMetrics>
             ReadParam(aMsg, aIter, &aResult->mDisableScrollingX) &&
             ReadParam(aMsg, aIter, &aResult->mDisableScrollingY) &&
             ReadParam(aMsg, aIter, &aResult->mUpdateScrollOffset) &&
-            ReadParam(aMsg, aIter, &aResult->mScrollGeneration));
+            ReadParam(aMsg, aIter, &aResult->mScrollGeneration) &&
+            ReadParam(aMsg, aIter, &aResult->mContentDescription));
   }
 };
 

gfx/layers/FrameMetrics.h

@@ -7,6 +7,7 @@
 #define GFX_FRAMEMETRICS_H
 
 #include <stdint.h>                     // for uint32_t, uint64_t
+#include <string>                       // for std::string
 #include "Units.h"                      // for CSSRect, CSSPixel, etc
 #include "mozilla/gfx/BasePoint.h"      // for BasePoint
 #include "mozilla/gfx/Rect.h"           // for RoundedIn
@@ -70,6 +71,8 @@ public:
 
   bool operator==(const FrameMetrics& aOther) const
   {
+    // mContentDescription is not compared on purpose as it's only used
+    // for debugging.
     return mCompositionBounds.IsEqualEdges(aOther.mCompositionBounds) &&
            mDisplayPort.IsEqualEdges(aOther.mDisplayPort) &&
            mCriticalDisplayPort.IsEqualEdges(aOther.mCriticalDisplayPort) &&
@@ -336,6 +339,16 @@ public:
     return mScrollGeneration;
   }
 
+  const std::string& GetContentDescription() const
+  {
+    return mContentDescription;
+  }
+
+  void SetContentDescription(const std::string& aContentDescription)
+  {
+    mContentDescription = aContentDescription;
+  }
+
 private:
   // New fields from now on should be made private and old fields should
   // be refactored to be private.
@@ -350,6 +363,10 @@ private:
   bool mUpdateScrollOffset;
   // The scroll generation counter used to acknowledge the scroll offset update.
   uint32_t mScrollGeneration;
+
+  // A description of the content element corresponding to this frame.
+  // This is empty unless the apz.printtree pref is turned on.
+  std::string mContentDescription;
 };
 
 /**

gfx/layers/composite/APZCTreeManager.cpp

@@ -16,6 +16,7 @@
 #include "mozilla/MouseEvents.h"
 #include "mozilla/mozalloc.h"           // for operator new
 #include "mozilla/TouchEvents.h"
+#include "mozilla/Preferences.h"        // for Preferences
 #include "nsDebug.h"                    // for NS_WARNING
 #include "nsPoint.h"                    // for nsIntPoint
 #include "nsThreadUtils.h"              // for NS_IsMainThread
@@ -30,12 +31,16 @@ namespace layers {
 
 float APZCTreeManager::sDPI = 160.0;
 
+// Pref that enables printing of the APZC tree for debugging.
+static bool gPrintApzcTree = false;
+
 APZCTreeManager::APZCTreeManager()
     : mTreeLock("APZCTreeLock"),
       mTouchCount(0)
 {
   MOZ_ASSERT(NS_IsMainThread());
   AsyncPanZoomController::InitializeGlobalState();
+  Preferences::AddBoolVarCache(&gPrintApzcTree, "apz.printtree", gPrintApzcTree);
 }
 
 APZCTreeManager::~APZCTreeManager()

layout/base/nsDisplayList.cpp

@@ -592,6 +592,17 @@ static void AdjustForScrollBars(ScreenIntRect& aToAdjust, nsIScrollableFrame* aS
   }
 }
 
+static bool gPrintApzcTree = false;
+
+static bool GetApzcTreePrintPref() {
+  static bool initialized = false;
+  if (!initialized) {
+    Preferences::AddBoolVarCache(&gPrintApzcTree, "apz.printtree", gPrintApzcTree);
+    initialized = true;
+  }
+  return gPrintApzcTree;
+}
+
 static void RecordFrameMetrics(nsIFrame* aForFrame,
                                nsIFrame* aScrollFrame,
                                const nsIFrame* aReferenceFrame,
@@ -733,6 +744,14 @@ static void RecordFrameMetrics(nsIFrame* aForFrame,
     AdjustForScrollBars(metrics.mCompositionBounds, scrollableFrame);
   }
 
+  if (GetApzcTreePrintPref()) {
+    if (nsIContent* content = frameForCompositionBoundsCalculation->GetContent()) {
+      nsAutoString contentDescription;
+      content->Describe(contentDescription);
+      metrics.SetContentDescription(NS_LossyConvertUTF16toASCII(contentDescription).get());
+    }
+  }
+
   metrics.mPresShellId = presShell->GetPresShellId();
 
   // If the scroll frame's content is marked 'scrollgrab', record this

modules/libpref/src/init/all.js

@@ -308,6 +308,9 @@ pref("media.audio_data.enabled", false);
 // 2 = STICKY (Allow lock to be broken, with hysteresis)
 pref("apz.axis_lock_mode", 0);
 
+// Whether to print the APZC tree for debugging
+pref("apz.printtree", false);
+
 #ifdef XP_MACOSX
 // Whether to run in native HiDPI mode on machines with "Retina"/HiDPI display;
 //   <= 0 : hidpi mode disabled, display will just use pixel-based upscaling