[Async] Transition animations break with experimental.async enabled due to premature {#if} evaluation

[MaxGurish]

Describe the bug

First, a quick note: the unusual implementation in the REPL is necessary for bind:offsetWidth to work correctly in my use case, though this detail isn’t directly relevant to the issue at hand.

Issue Summary:
When the counter updates (triggering a re-creation of the {#key} block), the {#if} block immediately applies the condition using the latest value of the state - bypassing the intermediate or expected rendering steps.

• If I use the key directly (without an update counter), the logic works as expected—but this approach breaks bind:offsetWidth, which is why I can’t use it.
• Disabling experimental.async also resolves the issue and restores correct behavior.

This suggests a timing or scheduling inconsistency in how reactive updates and keyed block re-instantiation interact when experimental.async is enabled.

Reproduction

https://svelte.dev/playground/d6d94071b1554fc4802b8e2371e438fe?version=5.46.1

Logs

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (16) x64 13th Gen Intel(R) Core(TM) i7-13620H
    Memory: 14.54 GB / 31.70 GB
  Binaries:
    Node: 22.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 11.2.0 - C:\Program Files\nodejs\npm.CMD
    bun: 1.2.12 - C:\Users\guris\.bun\bin\bun.EXE
  Browsers:
    Chrome: 143.0.7499.170
    Edge: Chromium (141.0.3537.85)
    Internet Explorer: 11.0.26100.7309

Severity

annoyance