Bug 1645607 - avoid changing plain text documents when entering reader mode, r=jaws

The issue in this bug was caused by reader mode replacing the 'pre' tag
that we use to wrap plaintext documents with a series of paragraphs.
This change affected the original document. Instead, we can put the
paragraphs in a clone of the original document, which is fine for
reader mode and avoids altering the original.

Differential Revision: https://phabricator.services.mozilla.com/D79713

toolkit/components/reader/ReaderMode.jsm

@@ -459,6 +459,9 @@ var ReaderMode = {
 
     let serializer = new XMLSerializer();
     let serializedDoc = serializer.serializeToString(doc);
+    // Explicitly null out doc to make it clear it might not be available from this
+    // point on.
+    doc = null;
 
     let options = {
       classesToPreserve: CLASSES_TO_PRESERVE,
@@ -476,10 +479,6 @@ var ReaderMode = {
       histogram.add(PARSE_ERROR_WORKER);
     }
 
-    // Explicitly null out doc to make it clear it might not be available from this
-    // point on.
-    doc = null;
-
     if (!article) {
       this.log("Worker did not return an article");
       histogram.add(PARSE_ERROR_NO_ARTICLE);
@@ -660,8 +659,11 @@ var ReaderMode = {
       }
       docFrag.append(pElem);
     }
-    preTag.parentNode.replaceChild(docFrag, preTag);
-    return doc;
+    // Clone the document to avoid the original document being affected
+    // (which shows up when exiting reader mode again).
+    let clone = doc.documentElement.cloneNode(true);
+    clone.querySelector("pre").replaceWith(docFrag);
+    return clone;
   },
 };