Issue 6803 - Implement readd snippet

lib/content/snippets.js

Index: lib/content/snippets.js
===================================================================
--- a/lib/content/snippets.js
+++ b/lib/content/snippets.js
@@ -115,16 +115,35 @@
     {
       element.style.setProperty("display", "none", "important");
     }
   })
   .observe(element, {attributes: true, attributeFilter: ["style"]});
 }
 
 /**
+ * Observes changes to a DOM node using a <code>MutationObserver</code>.
+ *
+ * @param {Node} target The DOM node to observe for changes.
+ * @param {MutationObserverInit?} [options] Options that describe what DOM
+ *   mutations should be reported to the callback.
+ * @param {function} callback A function that will be called on each DOM
+ *   mutation, taking a <code>MutationRecord</code> as its parameter.
+ */
+function observe(target, options, callback)
+{
+  new MutationObserver(mutations =>
+  {
+    for (let mutation of mutations)
+      callback(mutation);
+  })
+  .observe(target, options);
+}
+
+/**
  * Logs its arguments to the console. This may be used for testing and
  * debugging.
  *
  * @param {...*} [args] The arguments to log.
  */
 function log(...args)
 {
   console.log(...args);
@@ -235,8 +254,39 @@
 
       return root;
     }
   });
 }
 
 exports["hide-if-shadow-contains"] = makeInjector(hideIfShadowContains,
                                                   hideElement);
+
+/**
+ * Readds to the document any removed HTML elements that match a CSS selector.
+ *
+ * @param {string} selector The CSS selector that a removed HTML element should
+ *   match for it to be added back.
+ * @param {string?} [parentSelector] The CSS selector that a removed HTML
+ *   element's former parent should match for it to be added back.
+ */
+function readd(selector, parentSelector = null)
+{
+  observe(document, {childList: true, subtree: true}, mutation =>
+  {
+    if (mutation.removedNodes &&
+        (!parentSelector || (mutation.target instanceof Element &&
+                             mutation.target.matches(parentSelector))))
+    {
+      for (let node of mutation.removedNodes)
+      {
+        if (node instanceof HTMLElement && node.matches(selector))
+        {
+          // We don't have the location of the element in its former parent,
+          // but it's usually OK to just add it at the end.
+          mutation.target.appendChild(node);
+        }
+      }
+    }
+  });
+}
+
+exports.readd = readd;