Svelte 5: Displaying component-bound prop causes ERR_SVELTE_HYDRATION_MISMATCH

[ciscoheat]

Describe the bug

When binding a prop to a component prop and displaying it in certain circumstances, in the browser there is a ERR_SVELTE_HYDRATION_MISMATCH error.

It varies quite a lot when it fails, so the repo gives the details in code.

Reproduction

https://github.com/ciscoheat/svelte5-bind-hydrate

Logs

ERR_SVELTE_HYDRATION_MISMATCH: Hydration failed because the initial UI does not match what was rendered on the server. TypeError: Cannot read properties of undefined (reading '__nodeValue')
    at text (render.js:444:30)
    at render.js:434:22
    at execute_signal_fn (runtime.js:360:6)
    at execute_effect (runtime.js:538:29)
    at schedule_effect (runtime.js:634:4)
    at internal_create_effect (runtime.js:1383:3)
    at render_effect (runtime.js:1513:9)
    at Module.text_effect (render.js:434:2)
    at +page.svelte:10:20
    at render.js:1523:5

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (16) x64 Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
    Memory: 44.00 GB / 63.93 GB
  Binaries:
    Node: 21.4.0 - C:\Program Files\nodejs\node.EXE       
    Yarn: 1.22.21 - ~\AppData\Local\pnpm\yarn.CMD
    npm: 9.3.1 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.15.1 - ~\AppData\Local\pnpm\pnpm.CMD
  Browsers:
    Edge: Chromium (121.0.2277.106)
    Internet Explorer: 11.0.19041.3636
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.0.0-next.50

Severity

blocking an upgrade

[ciscoheat]

Added some more test cases to the repo.

[dummdidumm]

The minimum reproducible is just {''} - the empty string cases problems (and [] is becoming the empty string).

Out of curiousity: This kind of binding pattern is likely an error in Svelte 5 runes mode (i.e. having a undefined value bound to a property with a fallback value). What's the use case for this, and can you work around it by just initializing the value in the parent?

[ciscoheat]

There's no special use case, I think I was copying a component from a failing test, and added some text. Didn't see that the initialization is backwards! I can certainly initialize it in the parent instead.