From 6905267b2878930a110837f6511ea39bcc833c5e Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Wed, 20 Sep 2017 08:26:53 +1000
Subject: [PATCH] Bug 1400540 - Decide style backend type based on principal.
 r=bholley

MozReview-Commit-ID: A4y9Shdbr7g

--HG--
extra : source : cdfe5e5e059273a1c7fc6506d6e0fecb2bb7affb
---
 dom/base/nsDocument.cpp | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 642960e6a155..265af0df07b9 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -13465,20 +13465,15 @@ nsIDocument::UpdateStyleBackendType()
 
 #ifdef MOZ_STYLO
   if (nsLayoutUtils::StyloEnabled()) {
-    if (IsBeingUsedAsImage()) {
-      // Enable stylo for SVG-as-image.
+    // Disable stylo only for system principal. Other principals aren't
+    // able to use XUL by default, and the back door to enable XUL is
+    // mostly just for testing, which means they don't matter, and we
+    // shouldn't respect them at the same time.
+    // Note that, since tests can have XUL support, we still need to
+    // explicitly exclude XUL documents here.
+    if (!nsContentUtils::IsSystemPrincipal(NodePrincipal()) &&
+        !IsXULDocument()) {
       mStyleBackendType = StyleBackendType::Servo;
-    } else if (!mDocumentContainer) {
-      // Not docshell, assume Gecko. Various callers can end up setting this
-      // explicitly afterwards to inherit it in various situations.
-    } else if (!IsXULDocument() && IsContentDocument()) {
-      // Disable stylo for about: pages other than about:blank, since
-      // they tend to use unsupported selectors like XUL tree pseudos.
-      bool isAbout = false;
-      mDocumentURI->SchemeIs("about", &isAbout);
-      if (!isAbout || NS_IsAboutBlank(mDocumentURI)) {
-        mStyleBackendType = StyleBackendType::Servo;
-      }
     }
   }
 #endif