Bug 1259889 Part 2 - Load html.css lazily for pref changes. r=heycam

We need to re-evaluate html.css whenever "dom.details_element.enabled"
is changed to make the disclosure triangle for summary elements show up.
Reftests for details and summary will need the a live pref to work.

MozReview-Commit-ID: 9SN1fQBuwA7

--HG--
extra : rebase_source : c8eafde9d3a97908c44099219603f76087d484b9

layout/style/nsLayoutStylesheetCache.cpp

@@ -134,6 +134,11 @@ nsLayoutStylesheetCache::UASheet()
 StyleSheetHandle
 nsLayoutStylesheetCache::HTMLSheet()
 {
+  if (!mHTMLSheet) {
+    LoadSheetURL("resource://gre-resources/html.css",
+                 mHTMLSheet, eAgentSheetFeatures);
+  }
+
   return mHTMLSheet;
 }
 
@@ -319,8 +324,6 @@ nsLayoutStylesheetCache::nsLayoutStylesheetCache(StyleBackendType aType)
   // per-profile, since they're profile-invariant.
   LoadSheetURL("resource://gre-resources/counterstyles.css",
                mCounterStylesSheet, eAgentSheetFeatures);
-  LoadSheetURL("resource://gre-resources/html.css",
-               mHTMLSheet, eAgentSheetFeatures);
   LoadSheetURL("chrome://global/content/minimal-xul.css",
                mMinimalXULSheet, eAgentSheetFeatures);
   LoadSheetURL("resource://gre-resources/quirk.css",
@@ -375,6 +378,8 @@ nsLayoutStylesheetCache::For(StyleBackendType aType)
     //                               "layout.css.example-pref.enabled");
     Preferences::RegisterCallback(&DependentPrefChanged,
                                   "layout.css.grid.enabled");
+    Preferences::RegisterCallback(&DependentPrefChanged,
+                                  "dom.details_element.enabled");
   }
 
   return cache;
@@ -818,6 +823,7 @@ nsLayoutStylesheetCache::DependentPrefChanged(const char* aPref, void* aData)
                   gStyleCache_Servo ? &gStyleCache_Servo->sheet_ : nullptr);
 
   INVALIDATE(mUASheet);  // for layout.css.grid.enabled
+  INVALIDATE(mHTMLSheet); // for dom.details_element.enabled
 
 #undef INVALIDATE
 }

layout/style/res/html.css

@@ -771,13 +771,15 @@ video > .caption-box {
 }
 
 /* details & summary */
+/* Need to revert Bug 1259889 Part 2 when removing details preference. */
+@supports -moz-bool-pref("dom.details_element.enabled") {
+  details > summary::-moz-list-bullet {
+    list-style-type: disclosure-closed;
+  }
 
-details > summary::-moz-list-bullet {
+  details[open] > summary::-moz-list-bullet {
-  list-style-type: disclosure-closed;
+    list-style-type: disclosure-open;
-}
+  }
-
-details[open] > summary::-moz-list-bullet {
-  list-style-type: disclosure-open;
 }
 
 /* emulation of non-standard HTML <marquee> tag */