Since Laravel 5.8.16 the MailMessage class created by a Mail Notification implements the Renderable interface. This makes previewing it in a browser for testing much simpler. Rather than jumping through the hoops required before you can now do this:

Route::get('mail-preview', function () { $invoice = App\Invoice::find(1); return (new App\Notifications\InvoicePaid($invoice)) ->toMail(null); }); 

This is now much more in line with the ability to preview Mailable classes in the browser.

Thanks to Avraham Appel for making the pull request to make this possible.