fix: append_styles in an effect to make them available on mount

[paoloricciuti]

Closes #16472

Small technicality...I'm pretty sure the actual reproduction doesn't work anyway...but that was not working even before. The reason is that the reproduction is missing css inject. However this got me thinking...given that the style tag is meant to be inside the custom element shouldn't we always inject when building a custom element?

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Playground

pnpm add https://pkg.pr.new/svelte@16509

[Rich-Harris]

given that the style tag is meant to be inside the custom element shouldn't we always inject when building a custom element?

I think that was the intent:

svelte/packages/svelte/src/compiler/phases/2-analyze/index.js

Lines 503 to 504 in 39ee7cf

	custom_element: options.customElementOptions ?? options.customElement,
	inject_styles: options.css === 'injected' || options.customElement,

It looks like we forgot to handle the case where customElement is present in <svelte:options> but not in the global compiler config — changing it to this should fix it I think?

-inject_styles: options.css === 'injected' || options.customElement,
+inject_styles: options.css === 'injected' || !!options.customElementOptions || !!options.customElement,

[paoloricciuti]

Yeah it looks like, not at the desk right now so please don't merge, I'll fix this too when I'm back home (unless you beat me to it). I would probably extract custom_element to a variable and use that so that it's always in sync (even tho it's probably unlikely that we change how a custom element is created)

[Rich-Harris]

pushed a change, will hold off on merging until you've had a chance to review

[paoloricciuti]

pushed a change, will hold off on merging until you've had a chance to review

Looks perfect to me