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.

3 files changed, 10 insertions, 6 deletions

diff --git a/dom/base/nsContentSink.cpp b/dom/base/nsContentSink.cpp
index 1e6465a1bf..59f4a9f9a8 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 8ab2dab0bb..2e5cdac6f4 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 a41ae5e1d7..d9042c756d 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
   {