Describe the bug

In the documentation for the mount function, there's this paragraph:

Note that unlike calling new App(...) in Svelte 4, things like effects (including onMount callbacks, and action functions) will not run during mount. If you need to force pending effects to run (in the context of a test, for example) you can do so with flushSync().

Therefore you would expect to need to call flushSync() if you want the onMount callback to run for example, but I found a (simpler) solution from another issue (#15105 (comment)) which consisted of using await mount(...) instead of mount(...). I don't know if this is intended or not, but unless I'm mistaken, this isn't mentioned anywhere in the Svelte documentation, nor in the function's signature.

Perhaps it would be better to mention this somewhere in the documentation, and optionally to make the function return a promise (or create an async mount function that returns a promise) because the svelte language server is confused as to why I use await on a synchronous function.

Reproduction

I've created a minimal example here. You can see (in the console) that when mounting synchronously, the call to mount finishes before the onMount handler executes, and asynchronously, the onMount handler executes before the call to mount finishes because of the await.

Logs

System Info

System:
    OS: Linux 6.8 Linux Mint 22.1 (Xia)
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
    Memory: 5.70 GB / 15.50 GB
    Container: Yes
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 22.17.1 - ~/.nvm/versions/node/v22.17.1/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v22.17.1/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.17.1/bin/npm
    pnpm: 10.14.0 - ~/.local/share/pnpm/pnpm
    bun: 1.2.16 - /usr/local/bin/bun
  Browsers:
    Chrome: 131.0.6778.204
  npmPackages:
    svelte: ^5.0.0 => 5.38.0

Severity

annoyance