Transitions created by a transition function don't reverse correctly

[hgl]

Describe the bug

If a transition, created by a transition function, is quickly reversed, animation breaks.

Reproduction

1. go to https://svelte.dev/playground/23bcb97019b8434e87f24aeb2681f841?version=latest
2. click toggle, wait for the box to start sliding down
3. click toggle again quickly

the box should slide up and down smoothly, but it currently doesn't.

Logs

System Info

System:
    OS: macOS 15.5
    CPU: (12) arm64 Apple M2 Pro
    Memory: 60.95 MB / 32.00 GB
    Shell: 4.0.2 - /run/current-system/sw/bin/fish
  Binaries:
    Node: 24.1.0 - /nix/store/2q5an9rpdq4vhc5ag04ajxnzxxqsqchq-nodejs-24.1.0/bin/node
    npm: 11.3.0 - /nix/store/2q5an9rpdq4vhc5ag04ajxnzxxqsqchq-nodejs-24.1.0/bin/npm
    pnpm: 10.12.4 - /nix/store/y8hp276c80y9iprnisygix42h96qf9hg-pnpm-10.12.4/bin/pnpm
  Browsers:
    Chrome: 137.0.7151.120
    Safari: 18.5
    Safari Technology Preview: 26.0
  npmPackages:
    svelte: ^5.0.0 => 5.34.8

Severity

annoyance