Bug 1285474: Decide style system backend type for documents earlier. r=bholley

MozReview-Commit-ID: 7A4t3957CWe

dom/base/nsDocument.cpp

@@ -13653,14 +13653,27 @@ nsIDocument::ReportHasScrollLinkedEffect()
                                   "ScrollLinkedEffectFound2");
 }
 
-mozilla::StyleBackendType
-nsIDocument::GetStyleBackendType() const
+void
+nsIDocument::UpdateStyleBackendType()
 {
-  if (!mPresShell) {
+  MOZ_ASSERT(mStyleBackendType == StyleBackendType(0),
+             "no need to call UpdateStyleBackendType now");
 #ifdef MOZ_STYLO
-    NS_WARNING("GetStyleBackendType() called on document without a pres shell");
+  // XXX For now we use a Servo-backed style set only for (X)HTML documents
+  // in content docshells.  This should let us avoid implementing XUL-specific
+  // CSS features.  And apart from not supporting SVG properties in Servo
+  // yet, the root SVG element likes to create a style sheet for an SVG
+  // document before we have a pres shell (i.e. before we make the decision
+  // here about whether to use a Gecko- or Servo-backed style system), so
+  // we avoid Servo-backed style sets for SVG documents.
+  NS_ASSERTION(mDocumentContainer, "stylo: calling UpdateStyleBackendType "
+                                   "before we have a docshell");
+  mStyleBackendType =
+    nsLayoutUtils::SupportsServoStyleBackend(this) &&
+    mDocumentContainer ?
+      StyleBackendType::Servo :
+      StyleBackendType::Gecko;
+#else
+  mStyleBackendType = StyleBackendType::Gecko;
 #endif
-    return StyleBackendType::Gecko;
-  }
-  return mPresShell->StyleSet()->BackendType();
 }

dom/base/nsIDocument.h

@@ -1080,7 +1080,15 @@ public:
     return mCSSLoader;
   }
 
-  mozilla::StyleBackendType GetStyleBackendType() const;
+  mozilla::StyleBackendType GetStyleBackendType() const {
+    if (mStyleBackendType == mozilla::StyleBackendType(0)) {
+      const_cast<nsIDocument*>(this)->UpdateStyleBackendType();
+    }
+    MOZ_ASSERT(mStyleBackendType != mozilla::StyleBackendType(0));
+    return mStyleBackendType;
+  }
+
+  void UpdateStyleBackendType();
 
   bool IsStyledByServo() const {
     return GetStyleBackendType() == mozilla::StyleBackendType::Servo;
@@ -2916,6 +2924,10 @@ protected:
   // Our visibility state
   mozilla::dom::VisibilityState mVisibilityState;
 
+  // Whether this document has (or will have, once we have a pres shell) a
+  // Gecko- or Servo-backed style system.
+  mozilla::StyleBackendType mStyleBackendType;
+
   // True if BIDI is enabled.
   bool mBidiEnabled : 1;
   // True if a MathML element has ever been owned by this document.

layout/base/nsDocumentViewer.cpp

@@ -2165,27 +2165,6 @@ nsDocumentViewer::RequestWindowClose(bool* aCanClose)
   return NS_OK;
 }
 
-static StyleBackendType
-StyleBackendTypeForDocument(nsIDocument* aDocument, nsIDocShell* aContainer)
-{
-  MOZ_ASSERT(aDocument);
-
-  // XXX For now we use a Servo-backed style set only for (X)HTML documents
-  // in content docshells.  This should let us avoid implementing XUL-specific
-  // CSS features.  And apart from not supporting SVG properties in Servo
-  // yet, the root SVG element likes to create a style sheet for an SVG
-  // document before we have a pres shell (i.e. before we make the decision
-  // here about whether to use a Gecko- or Servo-backed style system), so
-  // we avoid Servo-backed style sets for SVG documents.
-
-  return nsPresContext::StyloEnabled() &&
-         aDocument->IsHTMLOrXHTML() &&
-         aContainer &&
-         aContainer->ItemType() == nsIDocShell::typeContent ?
-           StyleBackendType::Servo :
-           StyleBackendType::Gecko;
-}
-
 StyleSetHandle
 nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument)
 {
@@ -2194,8 +2173,7 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument)
   // this should eventually get expanded to allow for creating
   // different sets for different media
 
-  StyleBackendType backendType =
-    StyleBackendTypeForDocument(aDocument, mContainer);
+  StyleBackendType backendType = aDocument->GetStyleBackendType();
 
   StyleSetHandle styleSet;
   if (backendType == StyleBackendType::Gecko) {

layout/base/nsLayoutUtils.cpp

@@ -9365,3 +9365,11 @@ nsLayoutUtils::GetCumulativeApzCallbackTransform(nsIFrame* aFrame)
   }
   return delta;
 }
+
+/* static */ bool
+nsLayoutUtils::SupportsServoStyleBackend(nsIDocument* aDocument)
+{
+  return nsPresContext::StyloEnabled() &&
+         aDocument->IsHTMLOrXHTML() &&
+         static_cast<nsDocument*>(aDocument)->IsContentDocument();
+}

layout/base/nsLayoutUtils.h

@@ -2854,6 +2854,14 @@ public:
    */
   static CSSPoint GetCumulativeApzCallbackTransform(nsIFrame* aFrame);
 
+  /*
+   * Returns whether the given document supports being rendered with a
+   * Servo-backed style system.  This checks whether Stylo is enabled
+   * globally, that the document is an HTML document, and that it is
+   * being presented in a content docshell.
+   */
+  static bool SupportsServoStyleBackend(nsIDocument* aDocument);
+
 private:
   static uint32_t sFontSizeInflationEmPerLine;
   static uint32_t sFontSizeInflationMinTwips;

layout/style/Loader.h

@@ -201,19 +201,6 @@ public:
   NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(Loader)
   NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(Loader)
 
-  /**
-   * Returns the StyleBackendType that will be used for style sheets created
-   * by this Loader.  If SetStyleBackendType has been called, that value
-   * will be returned by GetStyleBackendType.  For Loaders created with the
-   * nsIDocument* constructor and which haven't had SetStyleBackendType
-   * called on them, the document's StyleBackendType will be returned.
-   */
-  StyleBackendType GetStyleBackendType() const;
-
-  void SetStyleBackendType(StyleBackendType aType) {
-    mStyleBackendType = Some(aType);
-  }
-
   void DropDocumentReference(); // notification that doc is going away
 
   void SetCompatibilityMode(nsCompatibility aCompatMode)
@@ -552,6 +539,8 @@ private:
   void DoSheetComplete(SheetLoadData* aLoadData, nsresult aStatus,
                        LoadDataArray& aDatasToNotify);
 
+  StyleBackendType GetStyleBackendType() const;
+
   struct Sheets {
     nsBaseHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey,
                     StyleSheetHandle::RefPtr,
@@ -588,6 +577,8 @@ private:
   nsCompatibility   mCompatMode;
   nsString          mPreferredSheet;  // title of preferred sheet
 
+  // Set explicitly when the Loader(StyleBackendType) constructor is used, or
+  // taken from the document when the Loader(nsIDocument*) constructor is used.
   mozilla::Maybe<StyleBackendType> mStyleBackendType;
 
   bool              mEnabled; // is enabled to load new styles

layout/style/StyleBackendType.h

@@ -14,7 +14,7 @@ namespace mozilla {
  */
 enum class StyleBackendType : int
 {
-  Gecko,
+  Gecko = 1,
   Servo
 };