fix: handle hydration mismatches in await blocks

[Rich-Harris]

closes #15704

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Playground

pnpm add https://pkg.pr.new/svelte@15708

[Rican7]

Oh wow, this is great! Thanks Rich! I appreciate it!

I would have tried to tackle this myself, but I wasn't sure where to start. Turns out it wasn't in Kit where I reported the issue. Whoops! 😅

[Rican7]

This is interesting. I was actually finding that the runtime was only having issue when I used a Component and an if (browser) conditional in the await block, rather than "normal" HTML. I have no idea why, though, and that may have just been due to my surrounding structure.

[Rich-Harris]

I think the component is sufficient but not necessary — basically anything that adds 'structure' will confuse the hydration algorithm if there's a mismatch, and the if block does the same thing. For test cases it's handy to keep things in a single component where possible, so I always try to whittle down

[Rican7]

Yea, that makes total sense. I'm just surprised this recreated the issue. Thanks for the fix!