src/vs/workbench/contrib/webview/browser/webviewService.ts/WebviewService/registerNewWebview
class WebviewService:	protected registerNewWebview(webview: IWebview) {
		this._webviews.add(webview);

		webview.onDidFocus(() => {
			this.updateActiveWebview(webview);
		});

		const onBlur = () => {
			if (this._activeWebview === webview) {
				this.updateActiveWebview(undefined);
			}
		};

		webview.onDidBlur(onBlur);
		webview.onDidDispose(() => {
			onBlur();
			this._webviews.delete(webview);
		});
	}