Good point to make a commit

tp-rtb-underline.js

@@ -0,0 +1,34 @@
+import { TpRtbBaseExtension } from './tp-rtb-base-extension.js';
+import Underline from '@tiptap/extension-underline';
+
+class TpRtbUnderline extends TpRtbBaseExtension {
+  constructor() {
+    super();
+    this.label = 'Underline';
+  }
+
+  getExtension() {
+    return Underline;
+  }
+
+  _handleClick() {
+    if (this.parentEditor && this.parentEditor.editor) {
+      this.parentEditor.editor.chain().focus().toggleUnderline().run();
+    }
+  }
+
+  _setupEditorListeners() {
+    const { editor } = this.parentEditor;
+    
+    // Update button state when selection changes
+    editor.on('selectionUpdate', () => {
+      this.active = editor.isActive('underline');
+    });
+    
+    editor.on('focus', () => {
+      this.active = editor.isActive('underline');
+    });
+  }
+}
+
+customElements.define('tp-rtb-underline', TpRtbUnderline);