Bug 1409446 - Handle nested display item scenarios. r=mstange

This handles some cases where a nested display item's clip chain
implicitly extends from the wrapper item's clip chain.

MozReview-Commit-ID: DmghxOWi81K

--HG--
extra : rebase_source : 8ec95df64fed247650baf8f5e0c868d1934aa6bc

gfx/layers/wr/ScrollingLayersHelper.cpp

@@ -184,6 +184,26 @@ ScrollingLayersHelper::RecurseAndDefineClip(nsDisplayItem* aItem,
       // than aChain->mParent.
       ancestorIds.second = Nothing();
     }
+  } else {
+    MOZ_ASSERT(!ancestorIds.second);
+    // If aChain->mASR is already the topmost scroll layer on the stack, but
+    // but there was another clip pushed *on top* of that ASR, then that clip
+    // shares the ASR, and we need to make our clip a child of that clip, which
+    // in turn will already be a descendant of the correct ASR.
+    // This covers the cases where e.g. the Gecko display list has nested items,
+    // and the clip chain on the nested item implicitly extends from the clip
+    // chain on the containing wrapper item. In this case the aChain->mParent
+    // pointer will be null for the nested item but the containing wrapper's
+    // clip will be on the stack already and we can pick it up from there.
+    // Another way of thinking about this is that if the clip chain were
+    // "fully completed" then aChain->mParent wouldn't be null but would point
+    // to the clip corresponding to mBuilder->TopmostClipId(), and we would
+    // have gone into the |aChain->mParent->mASR == aAsr| branch above.
+    FrameMetrics::ViewID scrollId = aChain->mASR ? nsLayoutUtils::ViewIDForASR(aChain->mASR) : FrameMetrics::NULL_SCROLL_ID;
+    if (mBuilder->TopmostScrollId() == scrollId && mBuilder->TopmostIsClip()) {
+      ancestorIds.first = Nothing();
+      ancestorIds.second = mBuilder->TopmostClipId();
+    }
   }
   // At most one of the ancestor pair should be defined here, and the one that
   // is defined will be the parent clip for the new clip that we're defining.

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -1169,5 +1169,14 @@ DisplayListBuilder::TopmostScrollId()
   return layers::FrameMetrics::NULL_SCROLL_ID;
 }
 
+bool
+DisplayListBuilder::TopmostIsClip()
+{
+  if (mClipStack.empty()) {
+    return false;
+  }
+  return mClipStack.back().is<wr::WrClipId>();
+}
+
 } // namespace wr
 } // namespace mozilla

gfx/webrender_bindings/WebRenderAPI.h

@@ -405,6 +405,8 @@ public:
   Maybe<wr::WrClipId> TopmostClipId();
   // Same as TopmostClipId() but for scroll layers.
   layers::FrameMetrics::ViewID TopmostScrollId();
+  // If the topmost item on the stack is a clip or a scroll layer
+  bool TopmostIsClip();
 
   // Try to avoid using this when possible.
   wr::WrState* Raw() { return mWrState; }