Bug 1389138 - Introduce a variant type to allow unifying the clip and scroll ids. r=mstange

This allows us to keep a single stack where we track both clip ids and
scroll ids with the correct interleaving order that they were pushed in.

MozReview-Commit-ID: DHHfR8JnnBw

--HG--
extra : rebase_source : f2f80e89a400d5b9982d138b29e534701b247072

gfx/layers/wr/ScrollingLayersHelper.cpp

@@ -21,7 +21,6 @@ ScrollingLayersHelper::ScrollingLayersHelper(WebRenderLayer* aLayer,
   : mLayer(aLayer)
   , mBuilder(&aBuilder)
   , mPushedLayerLocalClip(false)
-  , mClipsPushed(0)
 {
   if (!mLayer->WrManager()->AsyncPanZoomEnabled()) {
     // If APZ is disabled then we don't need to push the scrolling clips. We
@@ -93,7 +92,6 @@ ScrollingLayersHelper::ScrollingLayersHelper(nsDisplayItem* aItem,
   : mLayer(nullptr)
   , mBuilder(&aBuilder)
   , mPushedLayerLocalClip(false)
-  , mClipsPushed(0)
 {
   DefineAndPushChain(aItem->GetClipChain(), aBuilder, aStackingContext,
       aItem->Frame()->PresContext()->AppUnitsPerDevPixel(), aCache);
@@ -138,7 +136,7 @@ ScrollingLayersHelper::DefineAndPushChain(const DisplayItemClipChain* aChain,
   // Finally, push the clip onto the WR stack
   MOZ_ASSERT(clipId);
   aBuilder.PushClip(clipId.value());
-  mClipsPushed++;
+  mPushedClips.push_back(wr::ScrollOrClipId(clipId.value()));
 }
 
 bool
@@ -213,10 +211,12 @@ ScrollingLayersHelper::PushLayerClip(const LayerClip& aClip,
 ScrollingLayersHelper::~ScrollingLayersHelper()
 {
   if (!mLayer) {
-    // For layers-free mode
-    while (mClipsPushed > 0) {
+    // For layers-free mode.
+    while (!mPushedClips.empty()) {
+      wr::ScrollOrClipId id = mPushedClips.back();
+      MOZ_ASSERT(id.is<wr::WrClipId>());
       mBuilder->PopClip();
-      mClipsPushed--;
+      mPushedClips.pop_back();
     }
     return;
   }

gfx/layers/wr/ScrollingLayersHelper.h

@@ -51,7 +51,7 @@ private:
   WebRenderLayer* mLayer;
   wr::DisplayListBuilder* mBuilder;
   bool mPushedLayerLocalClip;
-  int mClipsPushed;
+  std::vector<wr::ScrollOrClipId> mPushedClips;
 };
 
 } // namespace layers

gfx/webrender_bindings/WebRenderTypes.h

@@ -6,6 +6,7 @@
 #ifndef GFX_WEBRENDERTYPES_H
 #define GFX_WEBRENDERTYPES_H
 
+#include "FrameMetrics.h"
 #include "mozilla/webrender/webrender_ffi.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/gfx/Matrix.h"
@@ -14,6 +15,7 @@
 #include "mozilla/layers/LayersTypes.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Range.h"
+#include "mozilla/Variant.h"
 #include "Units.h"
 #include "RoundedRect.h"
 #include "nsStyleConsts.h"
@@ -662,6 +664,8 @@ struct WrClipId {
   }
 };
 
+typedef Variant<layers::FrameMetrics::ViewID, WrClipId> ScrollOrClipId;
+
 } // namespace wr
 } // namespace mozilla