src/vs/workbench/contrib/webview/browser/overlayWebview.ts/OverlayWebview/postMessage
class OverlayWebview:	public async postMessage(message: any, transfer?: readonly ArrayBuffer[]): Promise<boolean> {
		if (this._webview.value) {
			return this._webview.value.postMessage(message, transfer);
		}

		if (this._isFirstLoad) {
			let resolve: (x: boolean) => void;
			const p = new Promise<boolean>(r => resolve = r);
			this._firstLoadPendingMessages.add({ message, transfer, resolve: resolve! });
			return p;
		}

		return false;
	}