Issue 5899 - Use CSS attribute selectors for collapsing media elements

lib/cssInjection.js

Index: lib/cssInjection.js
===================================================================
--- a/lib/cssInjection.js
+++ b/lib/cssInjection.js
@@ -94,31 +94,34 @@
   browser.tabs.removeCSS(tabId, {
     code: styleSheet,
     cssOrigin: "user",
     frameId,
     matchAboutBlank: true
   });
 }
 
-function updateFrameStyles(tabId, frameId, selectors, groupName)
+function updateFrameStyles(tabId, frameId, selectors, groupName, appendOnly)
 {
-  let styleSheet = null;
+  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);
 
+  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.
 
   // Add the new style sheet first to keep previously hidden elements from
   // reappearing momentarily.
   if (styleSheet && !addStyleSheet(tabId, frameId, styleSheet))
@@ -176,10 +179,10 @@
     response.inlineEmulated = true;
 
   return response;
 });
 
 port.on("elemhide.injectSelectors", (message, sender) =>
 {
   updateFrameStyles(sender.page.id, sender.frame.id, message.selectors,
-                    message.groupName);
+                    message.groupName, message.appendOnly);
 });