Bug 1290013 - Add a pref to disable stylo. r=bholley

MozReview-Commit-ID: DG9Q61OYlvl

layout/base/nsLayoutUtils.cpp

@@ -170,6 +170,9 @@ typedef nsStyleTransformMatrix::TransformReferenceBox TransformReferenceBox;
 /* static */ bool nsLayoutUtils::sInterruptibleReflowEnabled;
 /* static */ bool nsLayoutUtils::sSVGTransformBoxEnabled;
 /* static */ bool nsLayoutUtils::sTextCombineUprightDigitsEnabled;
+#ifdef MOZ_STYLO
+/* static */ bool nsLayoutUtils::sStyloEnabled;
+#endif
 
 static ViewID sScrollIdCounter = FrameMetrics::START_SCROLL_ID;
 
@@ -7840,6 +7843,10 @@ nsLayoutUtils::Initialize()
                                "svg.transform-box.enabled");
   Preferences::AddBoolVarCache(&sTextCombineUprightDigitsEnabled,
                                "layout.css.text-combine-upright-digits.enabled");
+#ifdef MOZ_STYLO
+  Preferences::AddBoolVarCache(&sStyloEnabled,
+                               "layout.css.servo.enabled");
+#endif
 
   for (auto& callback : kPrefCallbacks) {
     Preferences::RegisterCallbackAndCall(callback.func, callback.name);
@@ -9337,7 +9344,7 @@ nsLayoutUtils::GetCumulativeApzCallbackTransform(nsIFrame* aFrame)
 /* static */ bool
 nsLayoutUtils::SupportsServoStyleBackend(nsIDocument* aDocument)
 {
-  return nsPresContext::StyloEnabled() &&
+  return StyloEnabled() &&
          aDocument->IsHTMLOrXHTML() &&
          static_cast<nsDocument*>(aDocument)->IsContentDocument();
 }

layout/base/nsLayoutUtils.h

@@ -2421,6 +2421,17 @@ public:
     return sTextCombineUprightDigitsEnabled;
   }
 
+  // Stylo (the Servo backend for Gecko's style system) is generally enabled
+  // or disabled at compile-time. However, we provide the additional capability
+  // to disable it dynamically in stylo-enabled builds via a pref.
+  static bool StyloEnabled() {
+#ifdef MOZ_STYLO
+    return sStyloEnabled;
+#else
+    return false;
+#endif
+  }
+
   /**
    * See comment above "font.size.inflation.mappingIntercept" in
    * modules/libpref/src/init/all.js .
@@ -2861,6 +2872,9 @@ private:
   static bool sInterruptibleReflowEnabled;
   static bool sSVGTransformBoxEnabled;
   static bool sTextCombineUprightDigitsEnabled;
+#ifdef MOZ_STYLO
+  static bool sStyloEnabled;
+#endif
 
   /**
    * Helper function for LogTestDataForPaint().

layout/base/nsPresContext.h

@@ -1106,20 +1106,6 @@ public:
     mHasWarnedAboutTooLargeDashedOrDottedRadius = true;
   }
 
-  static bool StyloEnabled()
-  {
-    // Stylo (the Servo backend for Gecko's style system) is generally enabled
-    // or disabled at compile-time. However, we provide the additional capability
-    // to disable it dynamically in stylo-enabled builds via an environmental
-    // variable.
-#ifdef MOZ_STYLO
-    static bool disabled = PR_GetEnv("MOZ_DISABLE_STYLO");
-    return !disabled;
-#else
-    return false;
-#endif
-  }
-
 protected:
   friend class nsRunnableMethod<nsPresContext>;
   void ThemeChangedInternal();

modules/libpref/init/all.js

@@ -5500,3 +5500,8 @@ pref("osfile.reset_worker_delay", 30000);
 pref("dom.webkitBlink.dirPicker.enabled", true);
 pref("dom.webkitBlink.filesystem.enabled", true);
 #endif
+
+#ifdef MOZ_STYLO
+// Is the Servo-backed style system enabled?
+pref("layout.css.servo.enabled", true);
+#endif