No more node content in 5.34.4

[merlinaudio]

Describe the bug

I've updated to 5.34.4 and suddenly all of my nodes are missing their content. Inspect Element just shows <!--->.

Like, my app is almost blank. Most elements are there but images and aren't. I can't see any console logs that seem related.

Unfortunately I can't give a repro or paste the logs.

This is my svelte.config.js:

export default {
	preprocess: vitePreprocess(),

	compilerOptions: {
		modernAst: true, // removing this line doesn't fix the issue
	},
};

And vite.config.ts:

export default defineConfig(() => {
	return {
		plugins: [tailwindcss(), svelte()],
		build: {
			target: "safari15",
		},
	};
});

System Info

System:
    OS: macOS 26.0
    CPU: (12) arm64 Apple M2 Max
    Memory: 4.47 GB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Yarn: 1.22.22 - ~/.bun/bin/yarn
    npm: 11.3.0 - ~/.bun/bin/npm
    bun: 1.2.16 - ~/.bun/bin/bun
  Browsers:
    Chrome: 137.0.7151.104
    Safari: 19.0

Severity

blocking an upgrade

[merlinaudio]

I figured even though this issue is worth very little (because I cannot post logs/repro) I'd make it in case anyone else runs into this, or I find out more. Apologies for the little data!

[Conduitry]

If you can't give a repro, the issue will be closed.

[dummdidumm]

Which version is the first in which this appears? (also yes we need a reproduction)

[merlinaudio]

5.34.4 is the first version where this appears @dummdidumm.

5.34.3 is fine

I'll try to figure out how to repro

[heikkilamarko]

Hi. Not sure if this is related, but I can confirm that something in Svelte's reactivity broke after 5.34.3. In both 5.34.4 and 5.34.5, my app's state variables aren't updating correctly. If I navigate to another route and come back, I then see the updated values. Everything works fine with 5.34.3. I'm developing a SPA with SvelteKit.

[7nik]

@heikkilamarko just assumption, can it be #16100? It fixed the reactivity behavior of legacy components (ones without runes and without runes option) to match Svelte 4 behavior. So deep tracking doesn't work there.

[heikkilamarko]

Thanks! One of my components didn't use any runes, so it was in the legacy mode by default. Adding the following on top of that file fixed it:

<svelte:options runes={true} />

Is it possible to globally set this option via svelte.config.js?

[Thiagolino8]

Is it possible to globally set this option via svelte.config.js?

Yes

{
	compilerOptions: {
		runes: true
	}
}

[heikkilamarko]

Thanks! Without using a global option, it would be very easy to break an application. For example, suppose you have a component that uses a single $state rune, and you remove it. This causes the component to enter legacy mode, breaking reactivity with global stores etc. This happened in my case.

[dummdidumm]

@merlinaudio is your case the same?

How do we surface these kinds of bugs? We had a reactivity warning in place for reactivity loss in $: before but it was super noisy / false positive-y (though in this case we could only add those warnings if we detect that there's no legacy syntax)