Issue #1629 - Part 2: Implement the Explicitly Enabled flag.

This part of the bug was significantly complicated by the following major refactors:

https://bugzilla.mozilla.org/show_bug.cgi?id=1456435
https://bugzilla.mozilla.org/show_bug.cgi?id=1459498

As best as I can tell, we just need to implement the explicitly enabled
flag on every instance of GetStyleSheetInfo, make sure
aIsExplicitlyEnabled is false in every situation except the one where
the disabled content attribute is removed from a link element, and
enable alternate stylesheets if this flag is set on them. So we take the
explicitly enabled flag as an input to PrepareSheet, and also add it to
LoadStyleLink and LoadInlineStyle. I also decided not to defer loading of
alternate stylesheets that have been explicitly enabled.

dom/base/nsContentSink.cpp

@@ -789,13 +789,14 @@ nsContentSink::ProcessStyleLink(nsIContent* aElement,
   // If this is a fragment parser, we don't want to observe.
   // We don't support CORS for processing instructions
   bool isAlternate;
+  bool isExplicitlyEnabled;
   rv = mCSSLoader->LoadStyleLink(aElement, url, aTitle, aMedia, aAlternate,
                                  CORS_NONE, mDocument->GetReferrerPolicy(),
                                  integrity, mRunsToCompletion ? nullptr : this,
-                                 &isAlternate);
+                                 &isAlternate, &isExplicitlyEnabled);
   NS_ENSURE_SUCCESS(rv, rv);
   
-  if (!isAlternate && !mRunsToCompletion) {
+  if ((!isAlternate || isExplicitlyEnabled) && !mRunsToCompletion) {
     ++mPendingSheetCount;
     mScriptLoader->AddParserBlockingScriptExecutionBlocker();
   }

dom/base/nsStyleLinkElement.cpp

@@ -393,8 +393,9 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
   nsAutoString title, type, media;
   bool isScoped;
   bool isAlternate;
+  bool isExplicitlyEnabled;
 
-  GetStyleSheetInfo(title, type, media, &isScoped, &isAlternate);
+  GetStyleSheetInfo(title, type, media, &isScoped, &isAlternate, &isExplicitlyEnabled);
 
   if (!type.LowerCaseEqualsLiteral("text/css")) {
     return NS_OK;
@@ -425,7 +426,7 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
     // Parse the style sheet.
     rv = doc->CSSLoader()->
       LoadInlineStyle(thisContent, text, mLineNumber, title, media,
-                      scopeElement, aObserver, &doneLoading, &isAlternate);
+                      scopeElement, aObserver, &doneLoading, &isAlternate, &isExplicitlyEnabled);
   }
   else {
     nsAutoString integrity;
@@ -452,13 +453,14 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
     rv = doc->CSSLoader()->
       LoadStyleLink(thisContent, clonedURI, title, media, isAlternate,
                     GetCORSMode(), referrerPolicy, integrity,
-                    aObserver, &isAlternate);
+                    aObserver, &isAlternate, &isExplicitlyEnabled);
     if (NS_FAILED(rv)) {
       // Don't propagate LoadStyleLink() errors further than this, since some
       // consumers (e.g. nsXMLContentSink) will completely abort on innocuous
       // things like a stylesheet load being blocked by the security system.
       doneLoading = true;
       isAlternate = false;
+      isExplicitlyEnabled = false;
       rv = NS_OK;
     }
   }

dom/base/nsStyleLinkElement.h

@@ -98,7 +98,8 @@ protected:
                                  nsAString& aType,
                                  nsAString& aMedia,
                                  bool* aIsScoped,
-                                 bool* aIsAlternate) = 0;
+                                 bool* aIsAlternate,
+                                 bool* aIsExplicitlyEnabled) = 0;
 
   virtual mozilla::CORSMode GetCORSMode() const
   {

dom/html/HTMLLinkElement.cpp

@@ -126,7 +126,6 @@ HTMLLinkElement::SetMozDisabled(bool aDisabled)
   return rv.StealNSResult();
 }
 
-
 NS_IMPL_STRING_ATTR(HTMLLinkElement, Charset, charset)
 NS_IMPL_URI_ATTR(HTMLLinkElement, Href, href)
 NS_IMPL_STRING_ATTR(HTMLLinkElement, Hreflang, hreflang)
@@ -407,9 +406,14 @@ HTMLLinkElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
                                 aName == nsGkAtoms::disabled));
     }
   } else {
-    // Since removing href or rel makes us no longer link to a
+    // If the disabled attribute is removed from a link element, the 
-    // stylesheet, force updates for those too.
+    // stylesheet may be explicitly enabled.
     if (aNameSpaceID == kNameSpaceID_None) {
+      if (aName == nsGkAtoms::disabled) {
+        mExplicitlyEnabled = true;
+      }
+      // Since removing href or rel makes us no longer link to a
+      // stylesheet, force updates for those too.
       if (aName == nsGkAtoms::href ||
           aName == nsGkAtoms::rel ||
           aName == nsGkAtoms::title ||
@@ -508,13 +512,15 @@ HTMLLinkElement::GetStyleSheetInfo(nsAString& aTitle,
                                    nsAString& aType,
                                    nsAString& aMedia,
                                    bool* aIsScoped,
-                                   bool* aIsAlternate)
+                                   bool* aIsAlternate,
+                                   bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsScoped = false;
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString rel;
   GetAttr(kNameSpaceID_None, nsGkAtoms::rel, rel);
@@ -524,10 +530,16 @@ HTMLLinkElement::GetStyleSheetInfo(nsAString& aTitle,
     return;
   }
 
+  // Is the link disabled?
   if (Disabled()) {
     return;
   }
 
+  // Is it explicitly enabled?
+  if (mExplicitlyEnabled) {
+    *aIsExplicitlyEnabled = true;
+  }
+
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);
   title.CompressWhitespace();

dom/html/HTMLLinkElement.h

@@ -181,10 +181,18 @@ protected:
                                  nsAString& aType,
                                  nsAString& aMedia,
                                  bool* aIsScoped,
-                                 bool* aIsAlternate) override;
+                                 bool* aIsAlternate,
-protected:
+                                 bool* aIsExplicitlyEnabled) override;
+
   RefPtr<nsDOMTokenList> mRelList;
 
+  // The "explicitly enabled" flag. This flag is set whenever the 'disabled'
+  // attribute is explicitly unset, and makes alternate stylesheets not be
+  // disabled by default anymore.
+  //
+  // See https://github.com/whatwg/html/issues/3840#issuecomment-481034206.
+  bool mExplicitlyEnabled = false;
+
 private:
   RefPtr<ImportLoader> mImportLoader;
 };

dom/html/HTMLStyleElement.cpp

@@ -223,12 +223,14 @@ HTMLStyleElement::GetStyleSheetInfo(nsAString& aTitle,
                                     nsAString& aType,
                                     nsAString& aMedia,
                                     bool* aIsScoped,
-                                    bool* aIsAlternate)
+                                    bool* aIsAlternate,
+                                    bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);

dom/html/HTMLStyleElement.h

@@ -88,7 +88,8 @@ protected:
                          nsAString& aType,
                          nsAString& aMedia,
                          bool* aIsScoped,
-                         bool* aIsAlternate) override;
+                         bool* aIsAlternate,
+                         bool* aIsExplicitlyEnabled) override;
   /**
    * Common method to call from the various mutation observer methods.
    * aContent is a content node that's either the one that changed or its

dom/svg/SVGStyleElement.cpp

@@ -271,9 +271,11 @@ SVGStyleElement::GetStyleSheetInfo(nsAString& aTitle,
                                    nsAString& aType,
                                    nsAString& aMedia,
                                    bool* aIsScoped,
-                                   bool* aIsAlternate)
+                                   bool* aIsAlternate,
+                                   bool* aIsExplicitlyEnabled)
 {
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);

dom/svg/SVGStyleElement.h

@@ -95,7 +95,8 @@ protected:
                          nsAString& aType,
                          nsAString& aMedia,
                          bool* aIsScoped,
-                         bool* aIsAlternate) override;
+                         bool* aIsAlternate,
+                         bool* aIsExplicitlyEnabled) override;
   virtual CORSMode GetCORSMode() const override;
 
   /**

dom/xml/XMLStylesheetProcessingInstruction.cpp

@@ -131,13 +131,15 @@ XMLStylesheetProcessingInstruction::GetStyleSheetInfo(nsAString& aTitle,
                                                       nsAString& aType,
                                                       nsAString& aMedia,
                                                       bool* aIsScoped,
-                                                      bool* aIsAlternate)
+                                                      bool* aIsAlternate,
+                                                      bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsScoped = false;
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   // xml-stylesheet PI is special only in prolog
   if (!nsContentUtils::InProlog(this)) {

dom/xml/XMLStylesheetProcessingInstruction.h

@@ -82,7 +82,8 @@ protected:
                          nsAString& aType,
                          nsAString& aMedia,
                          bool* aIsScoped,
-                         bool* aIsAlternate) override;
+                         bool* aIsAlternate,
+                         bool* aIsExplicitlyEnabled) override;
   virtual nsGenericDOMDataNode* CloneDataNode(mozilla::dom::NodeInfo *aNodeInfo,
                                               bool aCloneText) const override;
 };

layout/style/Loader.cpp

@@ -1278,7 +1278,8 @@ Loader::PrepareSheet(StyleSheet* aSheet,
                      const nsSubstring& aMediaString,
                      nsMediaList* aMediaList,
                      Element* aScopeElement,
-                     bool isAlternate)
+                     bool isAlternate,
+                     bool isExplicitlyEnabled)
 {
   NS_PRECONDITION(aSheet, "Must have a sheet!");
 
@@ -1307,7 +1308,7 @@ Loader::PrepareSheet(StyleSheet* aSheet,
   sheet->SetMedia(mediaList);
 
   sheet->SetTitle(aTitle);
-  sheet->SetEnabled(!isAlternate);
+  sheet->SetEnabled(!isAlternate || isExplicitlyEnabled);
   sheet->SetScopeElement(aScopeElement);
 }
 
@@ -1985,7 +1986,8 @@ Loader::LoadInlineStyle(nsIContent* aElement,
                         Element* aScopeElement,
                         nsICSSLoaderObserver* aObserver,
                         bool* aCompleted,
-                        bool* aIsAlternate)
+                        bool* aIsAlternate,
+                        bool* aIsExplicitlyEnabled)
 {
   LOG(("css::Loader::LoadInlineStyle"));
   NS_ASSERTION(mParsingDatas.Length() == 0, "We're in the middle of a parse?");
@@ -2017,8 +2019,9 @@ Loader::LoadInlineStyle(nsIContent* aElement,
                "Inline sheets should not be cached");
 
   LOG(("  Sheet is alternate: %d", *aIsAlternate));
+  LOG(("  Sheet is explicitly enabled: %d", *aIsExplicitlyEnabled));
 
-  PrepareSheet(sheet, aTitle, aMedia, nullptr, aScopeElement, *aIsAlternate);
+  PrepareSheet(sheet, aTitle, aMedia, nullptr, aScopeElement, *aIsAlternate, *aIsExplicitlyEnabled);
 
   if (aElement->HasFlag(NODE_IS_IN_SHADOW_TREE)) {
     ShadowRoot* containingShadow = aElement->GetContainingShadow();
@@ -2059,7 +2062,8 @@ Loader::LoadStyleLink(nsIContent* aElement,
                       ReferrerPolicy aReferrerPolicy,
                       const nsAString& aIntegrity,
                       nsICSSLoaderObserver* aObserver,
-                      bool* aIsAlternate)
+                      bool* aIsAlternate,
+                      bool* aIsExplicitlyEnabled)
 {
   LOG(("css::Loader::LoadStyleLink"));
   NS_PRECONDITION(aURL, "Must have URL to load");
@@ -2112,8 +2116,9 @@ Loader::LoadStyleLink(nsIContent* aElement,
   NS_ENSURE_SUCCESS(rv, rv);
 
   LOG(("  Sheet is alternate: %d", *aIsAlternate));
+  LOG(("  Sheet is explicitly enabled: %d", *aIsExplicitlyEnabled));
 
-  PrepareSheet(sheet, aTitle, aMedia, nullptr, nullptr, *aIsAlternate);
+  PrepareSheet(sheet, aTitle, aMedia, nullptr, nullptr, *aIsAlternate, *aIsExplicitlyEnabled);
 
   rv = InsertSheetInDoc(sheet, aElement, mDocument);
   NS_ENSURE_SUCCESS(rv, rv);
@@ -2138,9 +2143,10 @@ Loader::LoadStyleLink(nsIContent* aElement,
                                           aObserver, principal, requestingNode);
   NS_ADDREF(data);
 
-  // If we have to parse and it's an alternate non-inline, defer it
+  // If we have to parse and it's an alternate non-inline, defer it unless
+  // it's explicitly enabled.
   if (aURL && state == eSheetNeedsParser && mSheets->mLoadingDatas.Count() != 0 &&
-      *aIsAlternate) {
+      *aIsAlternate && !*aIsExplicitlyEnabled) {
     LOG(("  Deferring alternate sheet load"));
     URIPrincipalReferrerPolicyAndCORSModeHashKey key(data->mURI,
                                                      data->mLoaderPrincipal,
@@ -2267,6 +2273,7 @@ Loader::LoadChildSheet(StyleSheet* aParentSheet,
     state = eSheetComplete;
   } else {
     bool isAlternate;
+    bool isExplicitlyEnabled;
     const nsSubstring& empty = EmptyString();
     // For now, use CORS_NONE for child sheets
     rv = CreateSheet(aURL, nullptr, principal,
@@ -2277,7 +2284,7 @@ Loader::LoadChildSheet(StyleSheet* aParentSheet,
                      false, empty, state, &isAlternate, &sheet);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    PrepareSheet(sheet, empty, empty, aMedia, nullptr, isAlternate);
+    PrepareSheet(sheet, empty, empty, aMedia, nullptr, isAlternate, isExplicitlyEnabled);
   }
 
   rv = InsertChildSheet(sheet, aParentSheet, aParentRule);
@@ -2390,6 +2397,7 @@ Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
 
   StyleSheetState state;
   bool isAlternate;
+  bool isExplicitlyEnabled;
   RefPtr<StyleSheet> sheet;
   bool syncLoad = (aObserver == nullptr);
   const nsSubstring& empty = EmptyString();
@@ -2399,7 +2407,7 @@ Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
                    false, empty, state, &isAlternate, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  PrepareSheet(sheet, empty, empty, nullptr, nullptr, isAlternate);
+  PrepareSheet(sheet, empty, empty, nullptr, nullptr, isAlternate, isExplicitlyEnabled);
 
   if (state == eSheetComplete) {
     LOG(("  Sheet already complete"));

layout/style/Loader.h

@@ -230,6 +230,8 @@ public:
    * @param [out] aCompleted whether parsing of the sheet completed.
    * @param [out] aIsAlternate whether the stylesheet ended up being an
    *        alternate sheet.
+   * @param [out] aIsExplicitlyEnabled whether the stylesheet was explicitly
+   *        enabled by having the disabled attribute removed. 
    */
   nsresult LoadInlineStyle(nsIContent* aElement,
                            const nsAString& aBuffer,
@@ -239,7 +241,8 @@ public:
                            mozilla::dom::Element* aScopeElement,
                            nsICSSLoaderObserver* aObserver,
                            bool* aCompleted,
-                           bool* aIsAlternate);
+                           bool* aIsAlternate,
+			   bool* aIsExplicitlyEnabled);
 
   /**
    * Load a linked (document) stylesheet.  If a successful result is returned,
@@ -260,6 +263,8 @@ public:
    * @param [out] aIsAlternate whether the stylesheet actually ended up beinga
    *        an alternate sheet.  Note that this need not match
    *        aHasAlternateRel.
+   * @param [out] aIsExplicitlyEnabled whether the stylesheet was explicitly
+   *        enabled by having the disabled attribute removed.
    */
   nsresult LoadStyleLink(nsIContent* aElement,
                          nsIURI* aURL,
@@ -270,7 +275,8 @@ public:
                          ReferrerPolicy aReferrerPolicy,
                          const nsAString& aIntegrity,
                          nsICSSLoaderObserver* aObserver,
-                         bool* aIsAlternate);
+                         bool* aIsAlternate,
+			 bool* aIsExplicitlyEnabled);
 
   /**
    * Load a child (@import-ed) style sheet.  In addition to loading the sheet,
@@ -476,7 +482,8 @@ private:
                     const nsAString& aMediaString,
                     nsMediaList* aMediaList,
                     dom::Element* aScopeElement,
-                    bool isAlternate);
+                    bool isAlternate,
+		    bool isExplicitlyEnabled);
 
   nsresult InsertSheetInDoc(StyleSheet* aSheet,
                             nsIContent* aLinkingContent,