Fix serialization and mutation of <textarea> elements (#1351)

* Fix serialization and mutation of <textarea> elements taking account the duality that the value can be set in either the child node, or in the value _parameter_ (not attribute)

* Backwards compatibility: Bug fix and regression test for #112
 - this is to fix up 'historical' recordings, as duplicate textarea content should no longer be being created at record time
 - new test shows what the snapshot generated by previous versions of rrweb used to look like, hence 'bad'
 - original 0efe23f04a fix either didn't work or no longer works due to childNodes being appended subsequent to this part of the code
 - during review, we also verified that the `_cssText` case should still be handled okay, as there's currently no scenario where csstext is present with css child nodes of a <style>

* Masking: Fix that textarea values were being missed by the masking system if the value was recorded as a child node
 - I didn't notice that form.html was used in other tests, so lucky that I noticed that those tests also should have the 'pre value' masked out

* Simplify by always storing the textarea value in the `.value` attribute (from it's DOM property) and not as a childNode. It should still be rebuilt as a childNode rather than a property
---------

Authored-by: eoghanmurray <eoghan@getthere.ie>

packages/rrweb-snapshot/test/snapshot.test.ts

@@ -7,7 +7,7 @@ import {
   serializeNodeWithId,
   _isBlockedElement,
 } from '../src/snapshot';
-import { serializedNodeWithId } from '../src/types';
+import { serializedNodeWithId, elementNode } from '../src/types';
 import { Mirror } from '../src/utils';
 
 describe('absolute url to stylesheet', () => {
@@ -218,3 +218,42 @@ describe('scrollTop/scrollLeft', () => {
     });
   });
 });
+
+describe('form', () => {
+  const serializeNode = (node: Node): serializedNodeWithId | null => {
+    return serializeNodeWithId(node, {
+      doc: document,
+      mirror: new Mirror(),
+      blockClass: 'blockblock',
+      blockSelector: null,
+      maskTextClass: 'maskmask',
+      maskTextSelector: null,
+      skipChild: false,
+      inlineStylesheet: true,
+      maskTextFn: undefined,
+      maskInputFn: undefined,
+      slimDOMOptions: {},
+      newlyAddedElement: false,
+    });
+  };
+
+  const render = (html: string): HTMLTextAreaElement => {
+    document.write(html);
+    return document.querySelector('textarea')!;
+  };
+
+  it('should record textarea values once', () => {
+    const el = render(`<textarea>Lorem ipsum</textarea>`);
+    const sel = serializeNode(el) as elementNode;
+
+    // we serialize according to where the DOM stores the value, not how
+    // the HTML stores it (this is so that maskInputValue can work over
+    // inputs/textareas/selects in a uniform way)
+    expect(sel).toMatchObject({
+      attributes: {
+        value: 'Lorem ipsum',
+      },
+    });
+    expect(sel?.childNodes).toEqual([]); // shouldn't be stored in childNodes while in transit
+  });
+});