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.

13 files changed, 79 insertions, 31 deletions

diff --git a/dom/base/nsContentSink.cpp b/dom/base/nsContentSink.cpp
index 1e6465a1b..59f4a9f9a 100644
--- a/dom/base/nsContentSink.cpp
+++ b/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();
   }
diff --git a/dom/base/nsStyleLinkElement.cpp b/dom/base/nsStyleLinkElement.cpp
index 8ab2dab0b..2e5cdac6f 100644
--- a/dom/base/nsStyleLinkElement.cpp
+++ b/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;
     }
   }
diff --git a/dom/base/nsStyleLinkElement.h b/dom/base/nsStyleLinkElement.h
index a41ae5e1d..d9042c756 100644
--- a/dom/base/nsStyleLinkElement.h
+++ b/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
   {
diff --git a/dom/html/HTMLLinkElement.cpp b/dom/html/HTMLLinkElement.cpp
index ed83836aa..352cbc755 100644
--- a/dom/html/HTMLLinkElement.cpp
+++ b/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
-    // stylesheet, force updates for those too.
+    // If the disabled attribute is removed from a link element, the 
+    // 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();
diff --git a/dom/html/HTMLLinkElement.h b/dom/html/HTMLLinkElement.h
index e024d42bb..190213028 100644
--- a/dom/html/HTMLLinkElement.h
+++ b/dom/html/HTMLLinkElement.h
@@ -181,10 +181,18 @@ protected:
                                  nsAString& aType,
                                  nsAString& aMedia,
                                  bool* aIsScoped,
-                                 bool* aIsAlternate) override;
-protected:
+                                 bool* aIsAlternate,
+                                 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;
 };
diff --git a/dom/html/HTMLStyleElement.cpp b/dom/html/HTMLStyleElement.cpp
index 95cf025d1..c37e1a32d 100644
--- a/dom/html/HTMLStyleElement.cpp
+++ b/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);
diff --git a/dom/html/HTMLStyleElement.h b/dom/html/HTMLStyleElement.h
index 8b69c3f62..7d98a494a 100644
--- a/dom/html/HTMLStyleElement.h
+++ b/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
diff --git a/dom/svg/SVGStyleElement.cpp b/dom/svg/SVGStyleElement.cpp
index 22fb204df..7655c1198 100644
--- a/dom/svg/SVGStyleElement.cpp
+++ b/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);
diff --git a/dom/svg/SVGStyleElement.h b/dom/svg/SVGStyleElement.h
index 9b126e7e8..e637dfb18 100644
--- a/dom/svg/SVGStyleElement.h
+++ b/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;
 
   /**
diff --git a/dom/xml/XMLStylesheetProcessingInstruction.cpp b/dom/xml/XMLStylesheetProcessingInstruction.cpp
index 3d94e179b..43e45131a 100644
--- a/dom/xml/XMLStylesheetProcessingInstruction.cpp
+++ b/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)) {
diff --git a/dom/xml/XMLStylesheetProcessingInstruction.h b/dom/xml/XMLStylesheetProcessingInstruction.h
index 818b3392f..28061834a 100644
--- a/dom/xml/XMLStylesheetProcessingInstruction.h
+++ b/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;
 };
diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index 68a7be21e..c21199238 100644
--- a/layout/style/Loader.cpp
+++ b/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"));
diff --git a/layout/style/Loader.h b/layout/style/Loader.h
index 209783a80..4a3088b6b 100644
--- a/layout/style/Loader.h
+++ b/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,