Issue 5864 - Remove previous style sheet before adding one

lib/cssInjection.js

Index: lib/cssInjection.js
===================================================================
--- a/lib/cssInjection.js
+++ b/lib/cssInjection.js
@@ -25,33 +25,74 @@
 const {checkWhitelisted} = require("whitelisting");
 const {extractHostFromFrame} = require("url");
 const {port} = require("messaging");
 const devtools = require("devtools");
 
 const userStyleSheetsSupported = "extensionTypes" in browser &&
                                  "CSSOrigin" in browser.extensionTypes;
 
-function hideElements(tabId, frameId, selectors)
+function addStyleSheet(tabId, frameId, styleSheet)
 {
   browser.tabs.insertCSS(tabId, {
-    code: selectors.join(", ") + "{display: none !important;}",
+    code: styleSheet,
     cssOrigin: "user",
     frameId,
     matchAboutBlank: true,
     runAt: "document_start"
   });
 }
 
-port.on("elemhide.getSelectors", (msg, sender) =>
+function removeStyleSheet(tabId, frameId, styleSheet)
+{
+  browser.tabs.removeCSS(tabId, {
+    code: styleSheet,
+    cssOrigin: "user",
+    frameId,
+    matchAboutBlank: true
+  });
+}
+
+function updateFrameStyles(tabId, frameId, selectors, groupName)
+{
+  let styleSheet = null;
+  if (selectors.length > 0)
+    styleSheet = selectors.join(", ") + "{display: none !important;}";
+
+  let frame = ext.getFrame(tabId, frameId);
+  if (!frame.injectedStyleSheets)
+    frame.injectedStyleSheets = new Map();
+
+  let oldStyleSheet = frame.injectedStyleSheets.get(groupName);
+
+  // Ideally we would compare the old and new style sheets and skip this code
+  // if they're the same, but the old style sheet can be a leftover from a
+  // previous instance of the frame. We must add the new style sheet
+  // regardless.
+
+  // Add the new style sheet first to keep previously hidden elements from
+  // reappearing momentarily.
+  if (styleSheet)
+    addStyleSheet(tabId, frameId, styleSheet);
+
+  // Sometimes the old and new style sheets can be exactly the same. In such a
+  // case, do not remove the "old" style sheet, because it is in fact the new
+  // style sheet now.
+  if (oldStyleSheet && oldStyleSheet != styleSheet)
+    removeStyleSheet(tabId, frameId, oldStyleSheet);
+
+  frame.injectedStyleSheets.set(groupName, styleSheet);
+}
+
+port.on("elemhide.getSelectors", (message, sender) =>
 {
   let selectors = [];
   let emulatedPatterns = [];
   let trace = devtools && devtools.hasPanel(sender.page);
-  let inject = !userStyleSheetsSupported;
+  let inline = !userStyleSheetsSupported;
 
   if (!checkWhitelisted(sender.page, sender.frame,
                         RegExpFilter.typeMap.DOCUMENT |
                         RegExpFilter.typeMap.ELEMHIDE))
   {
     let hostname = extractHostFromFrame(sender.frame);
     let specificOnly = checkWhitelisted(sender.page, sender.frame,
                                         RegExpFilter.typeMap.GENERICHIDE);
@@ -60,22 +101,23 @@
       hostname,
       specificOnly ? ElemHide.SPECIFIC_ONLY : ElemHide.ALL_MATCHING
     );
 
     for (let filter of ElemHideEmulation.getRulesForDomain(hostname))
       emulatedPatterns.push({selector: filter.selector, text: filter.text});
   }
 
-  if (!inject && selectors.length > 0)
-    hideElements(sender.page.id, sender.frame.id, selectors);
+  if (!inline)
+    updateFrameStyles(sender.page.id, sender.frame.id, selectors);
 
-  let response = {trace, inject, emulatedPatterns};
-  if (trace || inject)
+  let response = {trace, inline, emulatedPatterns};
+  if (trace || inline)
     response.selectors = selectors;
 
   return response;
 });
 
-port.on("elemhide.injectSelectors", (msg, sender) =>
+port.on("elemhide.injectSelectors", (message, sender) =>
 {
-  hideElements(sender.page.id, sender.frame.id, msg.selectors);
+  updateFrameStyles(sender.page.id, sender.frame.id, message.selectors,
+                    message.groupName);
 });