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.

4 files changed, 31 insertions, 8 deletions

diff --git a/dom/html/HTMLLinkElement.cpp b/dom/html/HTMLLinkElement.cpp
index ed83836aac..352cbc7559 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 e024d42bbc..190213028e 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 95cf025d10..c37e1a32d7 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 8b69c3f625..7d98a494a8 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