Describe the bug

The onintrostart event fires when an element begins an animation specified with animate:.

This bug was introduced by #17567.

The best solution appears to be moving back the dispatchEvent() calls in packages/svelte/src/internal/client/dom/elements/transitions.js to the transition() function. Otherwise, we would probably need to add a parameter to animate(), indicating whether it was called by transition(), which may be a little dirty.

Maybe, we could add an on_begin callback parameter to animate(), which is how dispatchEvent() could be triggered to not break the fix of #14009. I can create a PR for this in case you like the solution :D

Reproduction

https://svelte.dev/playground/1b1bf2a52c2448a09917378711735c3d?version=5.55.1

Logs

System Info

System:
    OS: Linux 6.17 Ubuntu 25.10 25.10 (Questing Quokka)
    CPU: (6) x64 AMD Ryzen 5 4500U with Radeon Graphics
    Memory: 1008.57 MB / 6.67 GB
    Container: Yes
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 22.21.1 - /home/dabund24/.nvm/versions/node/v22.21.1/bin/node
    npm: 10.9.4 - /home/dabund24/.nvm/versions/node/v22.21.1/bin/npm
  Browsers:
    Firefox: 149.0
    Firefox Developer Edition: 149.0
  npmPackages:
    svelte: ^5.55.0 => 5.55.1

Severity

blocking an upgrade