Bug 1402094 - Blacklist about:reader from using stylo. r=bholley

MozReview-Commit-ID: 6cG1x1tcOwn

--HG--
extra : rebase_source : dce6a0b1828f8388952e5ff1993424dff782d3d5

dom/base/nsDocument.cpp

@@ -13608,6 +13608,27 @@ nsIDocument::ReportHasScrollLinkedEffect()
                                   "ScrollLinkedEffectFound2");
 }
 
+// URL-based blacklist for stylo.
+static bool
+ShouldUseGeckoBackend(nsIURI* aDocumentURI)
+{
+  if (!aDocumentURI) {
+    return false;
+  }
+  bool isScheme = false;
+  if (NS_SUCCEEDED(aDocumentURI->SchemeIs("about", &isScheme))) {
+    nsAutoCString path;
+    aDocumentURI->GetFilePath(path);
+    // about:reader requires support of :scope pseudo-class so we have
+    // to use Gecko backend for now. See bug 1402094.
+    // This should be fixed by bug 1204818.
+    if (path.EqualsLiteral("reader")) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void
 nsIDocument::UpdateStyleBackendType()
 {
@@ -13626,7 +13647,7 @@ nsIDocument::UpdateStyleBackendType()
     // Note that, since tests can have XUL support, we still need to
     // explicitly exclude XUL documents here.
     if (!nsContentUtils::IsSystemPrincipal(NodePrincipal()) &&
-        !IsXULDocument()) {
+        !IsXULDocument() && !ShouldUseGeckoBackend(mDocumentURI)) {
       mStyleBackendType = StyleBackendType::Servo;
     }
   }