src/vs/workbench/api/browser/mainThreadNotebookKernels.ts/MainThreadNotebookKernels/$postMessage
class MainThreadNotebookKernels:	async $postMessage(handle: number, editorId: string | undefined, message: any): Promise<boolean> {
		const tuple = this._kernels.get(handle);
		if (!tuple) {
			throw new Error('kernel already disposed');
		}
		const [kernel] = tuple;
		let didSend = false;
		for (const [editor] of this._editors) {
			if (!editor.hasModel()) {
				continue;
			}
			if (this._notebookKernelService.getMatchingKernel(editor.textModel).selected !== kernel) {
				// different kernel
				continue;
			}
			if (editorId === undefined) {
				// all editors
				editor.postMessage(message);
				didSend = true;
			} else if (editor.getId() === editorId) {
				// selected editors
				editor.postMessage(message);
				didSend = true;
				break;
			}
		}
		return didSend;
	}