Issue 7104 - Clear style sheets when frame structure is updated

lib/contentFiltering.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 96 matching lines @@
 
   if (!frame.state.injectedStyleSheets)
     frame.state.injectedStyleSheets = new Map();
 
   let oldStyleSheet = frame.state.injectedStyleSheets.get(groupName);
 
   if (appendOnly && oldStyleSheet)
     styleSheet = oldStyleSheet + styleSheet;
 
   // 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.
+  // if they're the same. But first we need to ensure that there are no edge
+  // cases that would cause the old style sheet to be a leftover from a
+  // previous instance of the frame (see issue #7180). For now, we 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))
     return false;
 
   // 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)
@@ skipping 134 matching lines @@
     return [...rulesFromStyleSheet(styleSheet)];
   }
 });
 
 fetch(browser.extension.getURL("/snippets.js"), {cache: "no-cache"})
 .then(response => response.ok ? response.text() : "")
 .then(text =>
 {
   snippetsLibrarySource = text;
 });