src/vs/workbench/contrib/webviewPanel/browser/webviewPanel.contribution.ts/WebviewPanelContribution/registerGroupListener
class WebviewPanelContribution:	private registerGroupListener(group: IEditorGroup): void {
		const listener = group.onWillOpenEditor(e => this.onEditorOpening(e.editor, group));

		Event.once(group.onWillDispose)(() => {
			listener.dispose();
		});
	}