[bug]: missing megadropdown animation

[alizedebray]

🐞 Describe the bug

The mageadropdown animation is sometimes missing.

🛠️ To Reproduce

Steps to reproduce the behavior:

1. On desktop, go to https://swisspost-design-system-next.netlify.app/iframe.html?viewMode=story&id=27a2e64d-55ba-492d-ab79-5f7c5e818498--portal
2. Open a megadropdown
3. Reduce screen width to reach tablet or mobile layout
4. Open the burger menu
5. Open a megadropdown again
6. The animation was not applied

[alizedebray]

@myrta2302 Can you please check if that's still happening with the new animation?

[myrta2302]

Closing: This will be resolved by #6758.