'mañana'.padStart(7, '1') => "1mañana" // ok
    'man\u0303ana'.padStart(7, '1') => "mañana" // oops

You are being disingenuous here. Those are different strings, with different lengths (try copying this into the console):

    'mañana'.length // 6
    'mañana'.length // 7

The latter has two stacked characters. These issues are inherent to Unicode and `padStart` is treating the strings correctly. If you need normalization, use the .normalize method you mentioned yourself.

This is a major improvement: double-wide and stacked characters have been there since ES3, but now the language is providing standard tools to work with them.