I'll give it another chance one of these days, though. I'm hearing a lot of news about it lately so I suppose there will be improvements and maybe the newer versions are closer to what I want.
I'm still on Firefox for a few reasons but I've been checking it out as it evolves, and been pleasantly surprised with progress over time.
There's a few things that I think are inherent to Chromium, that I doubt they'll ever get back; one of these is definitely MDI. But on the other hand, there's also a few things that I believed were Presto-specific (HTML renderer-related, e.g. text-selection of link text) that they have somehow managed to re-implement on top of Chromium, so never say never.
> Unfortunately, Vivaldi wasn't recreating that when I tried it.
Yeah, Vivaldi is not snappy. But... given the state of browser UI performance today (massively regressed from 8 years ago), it compares favourably to competitors. Which isn't to be sniffed at considering they've added so many things on top of the beast that is Chromium.
Despite the apparent bloat of the default experience, what it really does is just increases discoverability of the UI for new users. While things like gestures, keymaps, etc. still allow people to run a very slimmed down browser if desired.
This is basically the old Opera approach. Vivaldi was created by the Opera founders, is made up of ex-Opera staff, and has the goal of recreating an Opera 12-esque browser experience on top of the Chromium engine. If you liked Opera 12, you'll like their "bloated / OS-replacement" (also highly customisable, almost everything is removable) UI approach. If you didn't like Opera 12, you likely won't.
If you haven't tried the latest snapshots you're in for a treat: the UI now bundles a full calendar and mail client :D
I use Firefox as my daily driver today, but I was an Opera 12 user: the UI was bloated-by-default out of the box but the two important caveats were: (1) 100% customisability allowing you to streamline it easily, and (2) bloat never impacted performance at all (in fact Opera was the most performant browser back then for heavy tab use, even while the integrated mail client was indexing hundreds of incoming mails). I can see perf. being a bigger challenge for Vivaldi though since they have less control over the Chromium foundations... Curious to see how they get on with this.
> I've found nothing about how their sync protocol works
As far as I've seen they haven't released the source of their sync server[0] or protocol, but the mechanism seems similar enough to Chrome that they've likely stuck to using a similar protocol, just with their own server endpoint(s). It prompts me to use macOS keychain for sync encryption, as Chrome does, so this seems to strongly imply e2e.
The internals UI[1] also gives some nice data for the curious. It shows e2e as being enabled for me, but it looks like it isn't always (possibly only newer versions of Vivaldi support e2e and they need some kind of backward compat.)
[0] https://bifrost.vivaldi.com/
[1] vivaldi://sync-internals