Top level await prevents onMount, bind:this from running

[dangodai]

Describe the bug

When using a top level await inside of a <script> tag both onMount and elements with bind:this will not run, despite all promises in the script tag resolving. I suspect onMount and bind:this run at roughly the same time internally and are being blocked by a single issue.

Reproduction

Reproduction of both issues can be found here:

https://www.sveltelab.dev/695vn9wj60ay38q?files=.%2Fsrc%2Froutes%2F%2Bpage.svelte

Observe that

1. The button bound to myButton is undefined. This is shown both in the markup and by clicking the button to log the value of myButton
2. onMount has not run. This is shown both in the markup and by the absence of the log printed inside of onMount
3. Removing the await ... line causes both issues to go away

Oddly, when navigating to the second page /page2 (which you can navigate to using the buttons) the onMount function from the first page will finally run. Navigating back to / will result in onMount and bind:this functioning seemingly correctly.

Logs

System Info

System:
    OS: Linux 6.15 Arch Linux
    CPU: (12) x64 AMD Ryzen 5 2600 Six-Core Processor
    Memory: 6.02 GB / 15.54 GB
    Container: Yes
    Shell: 5.3.3 - /bin/bash
  Binaries:
    Node: 22.17.0 - ~/.nvm/versions/node/v22.17.0/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.17.0/bin/npm
    pnpm: 10.13.1 - ~/.local/share/pnpm/pnpm
  Browsers:
    Chromium: 139.0.7258.66
  npmPackages:
    svelte: ^5.38.0 => 5.38.0

Severity

blocking an upgrade (to async svelte)

[dangodai]

Just a guess, since I don't know how Svelte internals work, but I think this may be related to #16571 as effect.pending() is indeed stuck at 1 while this issue is happening.

[g-libardi]

Had the same issue when trying to use the experimental APIs with apache echarts.

[hfcRed]

When using an attachment, the element passed into the attachment function is also undefined if the component consuming the attachment has any of its parents contain a top level await

[dangodai]

Something changed here, possibly from #16571 being completed, but there still seems to be a lingering problem.

If you update my reproduction to svelte@5.38.6 you will see the same bug. However if you add {$effect.pending()} to the template then the bug magically disappears. This wasn't the case in the Svelte version when I originally posted this.

[dangodai]

@hfcRed There is also this issue: #16627

Could all be the same problem, but you might be interested in subscribing there too.

[vladshcherbin]

Any workaround or version with onMount working?