Bug 1546018 - parent the legend clip chains. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D29528

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

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -807,10 +807,11 @@ void DisplayListBuilder::PopStackingContext(bool aIsReferenceFrame) {
 }
 
 wr::WrClipChainId DisplayListBuilder::DefineClipChain(
-    const nsTArray<wr::WrClipId>& aClips, const wr::WrClipChainId* aParent) {
+    const nsTArray<wr::WrClipId>& aClips, bool aParentWithCurrentChain) {
   const uint64_t* parent = nullptr;
-  if (aParent && aParent->id != wr::ROOT_CLIP_CHAIN) {
-    parent = &aParent->id;
+  if (aParentWithCurrentChain &&
+      mCurrentSpaceAndClipChain.clip_chain != wr::ROOT_CLIP_CHAIN) {
+    parent = &mCurrentSpaceAndClipChain.clip_chain;
   }
   uint64_t clipchainId = wr_dp_define_clipchain(
       mWrState, parent, aClips.Elements(), aClips.Length());
@@ -1170,9 +1171,8 @@ void DisplayListBuilder::SuspendClipLeafMerging() {
     mSuspendedClipChainLeaf = mClipChainLeaf;
     mSuspendedSpaceAndClipChain = Some(mCurrentSpaceAndClipChain);
 
-    wr::WrClipChainId currentClipChainId{mCurrentSpaceAndClipChain.clip_chain};
     auto clipId = DefineClip(Nothing(), *mClipChainLeaf);
-    auto clipChainId = DefineClipChain({clipId}, &currentClipChainId);
+    auto clipChainId = DefineClipChain({clipId}, true);
 
     mCurrentSpaceAndClipChain.clip_chain = clipChainId.id;
     mClipChainLeaf = Nothing();

gfx/webrender_bindings/WebRenderAPI.h

@@ -394,7 +394,7 @@ class DisplayListBuilder final {
   void PopStackingContext(bool aIsReferenceFrame);
 
   wr::WrClipChainId DefineClipChain(const nsTArray<wr::WrClipId>& aClips,
-                                    const wr::WrClipChainId* aParent = nullptr);
+                                    bool aParentWithCurrentChain = false);
 
   wr::WrClipId DefineClip(
       const Maybe<wr::WrSpaceAndClip>& aParent, const wr::LayoutRect& aClipRect,

layout/forms/nsFieldSetFrame.cpp

@@ -196,7 +196,7 @@ bool nsDisplayFieldSetBorder::CreateWebRenderCommands(
       nsTArray<mozilla::wr::ComplexClipRegion> array{region};
 
       auto clip = aBuilder.DefineClip(Nothing(), layoutRect, &array, nullptr);
-      auto clipChain = aBuilder.DefineClipChain({clip});
+      auto clipChain = aBuilder.DefineClipChain({clip}, true);
       clipOut.emplace(aBuilder, clipChain);
     }
   } else {