Flatten threads: add UI components to messages

- A quote is added to messages to show which was the previous message in
  the reply chain.
- A "reply" button is added to messages to allow users to compose a
  comment which is added at the end of the reply chain.
- A bug is fixed in calculating the parent reply ID in the extra info
  object. Now parent reply means the first message in a reply chain,
  while previous reply means the previous message in the reply chain.

Bug: twpowertools:153
Change-Id: I699507ade52e80287dd634e61f835d53af6a904d
diff --git a/src/contentScripts/communityConsole/flattenThreads/components/ReplyButton.js b/src/contentScripts/communityConsole/flattenThreads/components/ReplyButton.js
new file mode 100644
index 0000000..681c433
--- /dev/null
+++ b/src/contentScripts/communityConsole/flattenThreads/components/ReplyButton.js
@@ -0,0 +1,99 @@
+import '@material/web/button/outlined-button.js';
+
+import {css, html, LitElement} from 'lit';
+import {waitFor} from 'poll-until-promise';
+
+import {SHARED_MD3_STYLES} from '../../../../common/styles/md3.js';
+import {getExtraInfoNodes} from '../flattenThreads.js';
+
+export default class TwptFlattenThreadReplyButton extends LitElement {
+  static properties = {
+    extraInfo: {type: Object},
+  };
+
+  static styles = [
+    SHARED_MD3_STYLES,
+    css`
+    md-outlined-button {
+      --md-outlined-button-container-shape: 0.25rem;
+      --md-outlined-button-container-height: 38px;
+    }
+    `,
+  ];
+
+  constructor() {
+    super();
+    this.extraInfo = {};
+  }
+
+  render() {
+    return html`
+      <md-outlined-button
+          label="Reply"
+          @click=${this.openReplyEditor}>
+      </md-outlined-button>
+    `;
+  }
+
+  #defaultReply(messagePayload) {
+    const quoteHeader = document.createElement('div');
+    const italics = document.createElement('i');
+    italics.textContent = this.extraInfo?.authorName + ' said:';
+    quoteHeader.append(italics);
+
+    const quote = document.createElement('blockquote');
+    quote.innerHTML = messagePayload;
+    getExtraInfoNodes(quote)?.forEach?.(node => {
+      node.parentNode.removeChild(node);
+    });
+
+    const br1 = document.createElement('br');
+    const br2 = document.createElement('br');
+
+    return [quoteHeader, quote, br1, br2];
+  }
+
+  openReplyEditor() {
+    const messageId = this.extraInfo?.id;
+    const messagePayload = document.querySelector(
+        '[data-twpt-message-id="' + messageId +
+        '"] .scTailwindThreadPostcontentroot html-blob');
+    if (!messagePayload) {
+      console.error('[flattenThreads] Payload not found.');
+      return;
+    }
+
+    const parentId = this.extraInfo?.parentId ?? this.extraInfo?.id;
+    const parentNodeReply =
+        document.querySelector('[data-twpt-message-id="' + parentId + '"]')
+            ?.closest?.('sc-tailwind-thread-message-message-card');
+    const parentNodeReplyButton = parentNodeReply?.querySelector?.(
+        '.scTailwindThreadMessageMessagecardadd-comment button');
+    if (!parentNodeReplyButton) {
+      // This is not critical: the reply button might already have been clicked
+      // (so it no longer exists), or the thread might be locked so replying is
+      // disabled and the button does'nt exist.
+      console.debug('[flattenThreads] Reply button not found.');
+      return;
+    }
+
+    // Click the reply button.
+    parentNodeReplyButton.click();
+
+    // Fill in the default reply text (it includes a quote of the message the
+    // user wishes to reply to).
+    waitFor(() => {
+      const editor =
+          parentNodeReply?.querySelector('sc-tailwind-thread-reply-editor');
+      if (editor) return Promise.resolve(editor);
+      return Promise.reject(new Error('Editor not found.'));
+    }, {interval: 75, timeout: 10 * 1000}).then(editor => {
+      const payload =
+          editor?.querySelector('.scTailwindSharedRichtexteditoreditor');
+
+      payload.prepend(...this.#defaultReply(messagePayload.innerHTML));
+    });
+  }
+}
+window.customElements.define(
+    'twpt-flatten-thread-reply-button', TwptFlattenThreadReplyButton);