PaneResizer misbehaves and throws error when using the experimental async feature

[Blade67]

Describe the bug

As the title suggests, the PaneResizer fails to handle drag events when awaiting states to resolve. This behaviour is however very inconsistent. In the reproduction project this behaviour resolves itself after a few seconds, but with larger projects, the behaviour fully breaks, outputting only a assertion error.

Reproduction

Repo: https://github.com/Blade67/paneforge-async-minimal-reprod

Logs

Assertion failed!
assert @ assert.js:5
adjustLayoutByDelta @ adjust-layout.js:81
(anonymous) @ paneforge.svelte.js:136
onMove @ paneforge.svelte.js:437
(anonymous) @ events.js:61
without_reactive_context @ shared.js:44
target_handler @ events.js:60Understand this error
assert.js:6 Uncaught Error: Assertion failed!
    at assert (assert.js:6:15)
    at adjustLayoutByDelta (adjust-layout.js:81:13)
    at paneforge.svelte.js:136:32
    at HTMLBodyElement.onMove (paneforge.svelte.js:437:17)
    at events.js:61:21
    at without_reactive_context (shared.js:44:10)
    at HTMLBodyElement.target_handler (events.js:60:11)
assert @ assert.js:6
adjustLayoutByDelta @ adjust-layout.js:81
(anonymous) @ paneforge.svelte.js:136
onMove @ paneforge.svelte.js:437
(anonymous) @ events.js:61
without_reactive_context @ shared.js:44
target_handler @ events.js:60Understand this error

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (16) x64 13th Gen Intel(R) Core(TM) i7-13620H
    Memory: 5.86 GB / 31.62 GB
  Binaries:
    Node: 24.0.2 - C:\nvm4w\nodejs\node.EXE
    npm: 11.3.0 - C:\nvm4w\nodejs\npm.CMD
    bun: 1.2.17 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (136.0.3240.76)

Severity

annoyance

[huntabyte]

It's likely an issue with Svelte then, as there should have been no breaking changes introduced in a minor.

[pedramamani]

Here's a more minimal repro: https://stackblitz.com/edit/pfas
In async contexts, this querySelector for the resize handler returns null.
I’ve been waiting a couple of weeks for a Svelte fix, but it hasn’t appeared in the recent patch releases.

[huntabyte]

Did you open an issue with Svelte yet for this?

[pedramamani]

No. Frankly I didn't think it would be helpful unless I isolated the root cause outside of Paneforge, which I couldn't.

[huntabyte]

Could be related to this:

[huntabyte]

I will try to find time to look into this on my end when I get the chance! Thanks for providing the minimal repro!

[pedramamani]

Could be related to this

Likely.

In the PaneGroupState constructor, removing the effects that reference domContext resolves the issue. Any reference to domContext in the callbacks and it breaks.

watch([() => this.opts.id.current, () => this.layout, () => this.panesArray], () => {
	// return updateResizeHandleAriaValues({
	// 	groupId: this.opts.id.current,
	// 	layout: this.layout,
	// 	panesArray: this.panesArray,
	// 	domContext: this.domContext,
	// })
})

$effect(() => {
  	return untrack(() => {
		// this.domContext.root // uncomment this to error
		// return this.#setResizeHandlerEventListeners()
	})
})

Hope this helps.

[DarthGigi]

Any updates on this?