Bug 959847. Part 2. Don't ignore "ignore viewport scrolling" on subdocuments. r=roc

This means that the layer we create in nsSubDocumentFrame::BuildDisplayList becomes the scrollable layer for the document and we early early from ScrollFrameHelper::BuildDisplayList before it gets to the nsDisplayScrollLayer creation.

layout/generic/nsSubDocumentFrame.cpp

@@ -373,6 +373,8 @@ nsSubDocumentFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
 
   nsRect dirty;
   bool haveDisplayPort = false;
+  bool ignoreViewportScrolling = false;
+  nsIFrame* savedIgnoreScrollFrame = nullptr;
   if (subdocRootFrame) {
     nsIFrame* rootScrollFrame = presShell->GetRootScrollFrame();
     nsIContent* content = rootScrollFrame ? rootScrollFrame->GetContent() : nullptr;
@@ -387,6 +389,13 @@ nsSubDocumentFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
       dirty = dirty.ConvertAppUnitsRoundOut(parentAPD, subdocAPD);
     }
 
+    ignoreViewportScrolling =
+      rootScrollFrame && presShell->IgnoringViewportScrolling();
+    if (ignoreViewportScrolling) {
+      savedIgnoreScrollFrame = aBuilder->GetIgnoreScrollFrame();
+      aBuilder->SetIgnoreScrollFrame(rootScrollFrame);
+    }
+
     aBuilder->EnterPresShell(subdocRootFrame, dirty);
   }
 
@@ -479,6 +488,10 @@ nsSubDocumentFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
 
   if (subdocRootFrame) {
     aBuilder->LeavePresShell(subdocRootFrame, dirty);
+
+    if (ignoreViewportScrolling) {
+      aBuilder->SetIgnoreScrollFrame(savedIgnoreScrollFrame);
+    }
   }
 
   if (aBuilder->IsForImageVisibility()) {