Fix thread jumping when embeds load

[jamesnvc]

Regression, presumably from when the logic for thread scrolling changed.

If one switches to a group with a long thread that has links in it, they'll see it starts scrolled to the bottom as expected, but then as the link previews load, the height changes & they end up scrolled up somewhat.

[jamesnvc]

There's a proposed change to add a resize observer which would be very useful for this, without needing weird hacks to figure out when things have loaded or whatever https://github.com/w3c/csswg-drafts/blob/master/resize-observer-1/explainer.md#example-keeping-chat-window-scrolled-to-the-bottom

[rafd]

We're already using a ResizeObserver polyfill: https://github.com/braidchat/braid/blob/master/src/braid/core/client/ui/views/thread.cljs#L112

[jamesnvc]

Ah, oops. I guess maybe the polyfill doesn't handle the size changing in the way it is here properly then? 😕

[rafd]

I think I introduced ResizeObserver when fixing the "jumping textarea" issue, which may have regressed the scroll-to! behaviour.

[jamesnvc]

Looking at the events that are being sent, it does look like some resizes aren't actually being observed properly, so maybe the polyfill just isn't sufficient yet.