Issue 6999 - Generate style sheets in background page

lib/contentFiltering.js

Index: lib/contentFiltering.js
===================================================================
--- a/lib/contentFiltering.js
+++ b/lib/contentFiltering.js
@@ -92,22 +92,19 @@
   browser.tabs.removeCSS(tabId, {
     code: styleSheet,
     cssOrigin: "user",
     frameId,
     matchAboutBlank: true
   });
 }
 
-function updateFrameStyles(tabId, frameId, selectors, groupName, appendOnly)
+function updateFrameStyles(tabId, frameId, styleSheet, groupName = "standard",
+                           appendOnly = false)
 {
-  let styleSheet = "";
-  if (selectors.length > 0)
-    styleSheet = createStyleSheet(selectors);
-
   let frame = ext.getFrame(tabId, frameId);
   if (!frame)
     return false;
 
   if (!frame.injectedStyleSheets)
     frame.injectedStyleSheets = new Map();
 
   let oldStyleSheet = frame.injectedStyleSheets.get(groupName);
@@ -189,16 +186,17 @@
     // tabs.insertCSS for why we catch any error here and simply
     // return a rejected promise.
     return Promise.reject(error);
   }
 }
 
 port.on("content.applyFilters", (message, sender) =>
 {
+  let styleSheet = "";
   let selectors = [];
   let emulatedPatterns = [];
   let trace = HitLogger.hasListener(sender.page.id);
   let inline = !userStyleSheetsSupported;
 
   let {elemhide, snippets} = message.filterTypes ||
                              {elemhide: true, snippets: true};
 
@@ -227,38 +225,44 @@
     }
 
     if (elemhide && !checkWhitelisted(sender.page, sender.frame, null,
                                       RegExpFilter.typeMap.ELEMHIDE))
     {
       let specificOnly = checkWhitelisted(sender.page, sender.frame, null,
                                           RegExpFilter.typeMap.GENERICHIDE);
       selectors = ElemHide.getSelectorsForDomain(docDomain, specificOnly);
+      styleSheet = createStyleSheet(selectors);
 
       for (let filter of ElemHideEmulation.getRulesForDomain(docDomain))
         emulatedPatterns.push({selector: filter.selector, text: filter.text});
     }
   }
 
   if (!inline && !updateFrameStyles(sender.page.id, sender.frame.id,
-                                    selectors, "standard"))
+                                    styleSheet))
   {
     inline = true;
   }
 
   let response = {trace, inline, emulatedPatterns};
-  if (trace || inline)
+
+  if (inline)
+    response.styleSheet = styleSheet;
+
+  if (trace)
     response.selectors = selectors;
 
   return response;
 });
 
 port.on("content.injectSelectors", (message, sender) =>
 {
-  updateFrameStyles(sender.page.id, sender.frame.id, message.selectors,
+  updateFrameStyles(sender.page.id, sender.frame.id,
+                    createStyleSheet(message.selectors),
                     message.groupName, message.appendOnly);
 });
 
 fetch(browser.extension.getURL("/snippets.js"), {cache: "no-cache"})
 .then(response => response.ok ? response.text() : "")
 .then(text =>
 {
   snippetsLibrarySource = text;