Ask HN: What's the complexity in fixing SharedArrayBuffer across major browsers?

SharedArrayBuffer was disabled by default in all major browsers on 5 January, 2018 in response to Spectre. Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer

10 months later, they are still not fixed yet except in Chrome: https://caniuse.com/#search=SharedArrayBuffer

What is the complexity in fixing it across other browsers?

How did Chrome 68 manage to fix it?