Using setMode('system') breaks things

[yellowby]

Describe the bug

It looks like using setMode('system') somehow breaks mode-watcher.

I have a typical three-button mode control: Light, System, and Dark. Everything worked fine until 1.0 release. But now the behavior of mode-watcher is unstable if the mode is switched sequentially via setMode and system value was used. mode.current stops reflecting the actual mode. However, reloading the page will apply the proper mode that was set last.

Reproduction

Just click through Light/System/Dark sequentially in both directions.

Demo: https://sveltekit-mode-watcher.vercel.app/
Source: https://github.com/yellowby/sveltekit-mode-watcher/blob/main/src/routes/%2Bpage.svelte

Logs

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (32) x64 AMD Ryzen 9 9950X3D 16-Core Processor
    Memory: 63.18 GB / 95.58 GB
  Binaries:
    Node: 22.16.0 - ~\AppData\Local\fnm_multishells\43252_1750065416485\node.EXE
    npm: 10.9.2 - ~\AppData\Local\fnm_multishells\43252_1750065416485\npm.CMD
    pnpm: 10.11.0 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (137.0.3296.52)

Severity

annoyance

[huntabyte]

Thanks for the reproduction. Will look into this one.

[huntabyte]

Would you mind posting a video showing the behavior? I'm having a difficult time reproducing on my end.

[huntabyte]

And is your system light or dark to begin with?

[demetriuszhomir]

@huntabyte This issue ain't mine, but I have the same thing happening.
Here's my video 👀

20250702-2128-16.5551371.mp4

[demetriuszhomir]

Fun fact, it seems like system mode auto-switching works well.
I.e. when the mode is system, I change my OS mode a 4+ times in a row — and it applies the dark class & removes it with no issues every time I switch the OS mode.

[yellowby]

@huntabyte My default system mode is light. I'm able to reproduce the issue on both Windows and macOS, as well as in different browsers. 100% reproduction in my demo seems to be straightforward. If your system is light, click the buttons in sequence - Light, System, Dark. In case of dark mode, do the same in reverse order - Dark, System, Light.
But... I'm unable to reproduce the issue on the mode-watcher docs.