message: Turn `MutationObserver` on/off during DOM connect/disconnect in `Tab.ts`.
diff: /*
  - * Copyright 2014-2016 Simon Edwards <simon@simonzone.com>
  + * Copyright 2014-2020 Simon Edwards <simon@simonzone.com>
  *
  * This source code is licensed under the MIT license which is detailed in the LICENSE.txt file.
  */
  -import {log, Logger, getLogger} from "extraterm-logging";
  +import { Logger, getLogger} from "extraterm-logging";
  import { CustomElement } from 'extraterm-web-component-decorators';
  /**
  @@ -24,11 +24,15 @@ export class Tab extends HTMLElement {
  this._mutationObserver = new MutationObserver( (mutations) => {
  this._applyDraggable();
  });
  - this._mutationObserver.observe(this, { childList: true });
  }
  connectedCallback(): void {
  this._applyDraggable();
  + this._mutationObserver.observe(this, { childList: true });
  + }
  +
  + disconnectedCallback(): void {
  + this._mutationObserver.disconnect();
  }
  private _applyDraggable(): void {
  