Рассматривали ли вы <fieldset>
?
import { Component } from '@angular/core';
@Component({
selector: 'my-app',
template: `
<fieldset>
<legend>{{readonlyPreface}}</legend>
<textarea
kendoTextArea
(valueChange)="forcedPreface($event)"
>{{myTextArea}}</textarea>
</fieldset>
<hr>
Combined: <textarea readonly>{{combined}}</textarea>
`
})
export class AppComponent {
combined: string;
readonlyPreface = "Diagnosis: ";
forcedPreface(event) {
this.combined = this.readonlyPreface + event;
}
}