I haven't tried it on iOS, but Twitter's web app is quite good on Android/Firefox and Android/Chrome. Not quite as good as native, and prone to developing lag if I don't occasionally restart the browser. But surprisingly good.
I use the web app on iOS. Feature parity is 100% for me because I don't use push notifications at app on Twitter. The performance is a little worse (mostly touch targeting is not as good) but once you get used to it it is not noticeable. This is an acceptable tradeoff to me because it allows me to block almost all promoted tweets with Safari content blockers. I also think the Twitter native app may do devious things that are less likely to be permitted by Safari.
No just focus. And I've run into issues even if I don't expand anything. Around 40% of pages have a point below which I can't scroll (I'm jumped to the top of the page if I try). And then there's the unspecified error you get on around 60% of page loads that requires a refresh to fix.
They talk about the process of building it here: https://blog.twitter.com/engineering/en_us/topics/open-sourc...