Maybe it's also a bug because every warning should be ignorable? Not sure.

If that one somehow ends up being deemed an intended behavior rather than a bug, then it should definitely get explained in the docs too.

I'm not sure yet how much of this is a bug.

This isn't a bug. Svelte's reactivity is triggered by assignment, so it won't re-render on mutations like array pushing.

We want arr.push(foo); arr = arr; to work, and this is a bug.

(Note sure if this is a feature request or a bug...)

This isn't really a bug, because if you have an async function that returns a function, it doesn't really return a function - it returns a promise. I don't remember whether the Svelte internals currently check for the onMount callback's return being a function or whether they check for it being truthy. Maybe there's some adjustment to be made there.

Sometimes, the line between 'bug' and 'feature' is a hard one to draw. Generally, a feature is anything that adds new behavior, while a bug is anything that causes incorrect behavior. Sometimes, the core team will have to make a judgment call.

We iterate to deliver features, so we often don't have functionality that people expect. For this reason, 'people could reasonably expect this functionality' does not make it a bug.