rtl mobile sidebar: fade in starts from middle of page

### Describe the bug

https://github.com/user-attachments/assets/63cdc556-db2b-47d9-bf83-75611394e57a

### Reproduction

https://stackblitz.com/edit/github-tmryg48h?file=src%2Froutes%2F%2Bpage.svelte,src%2Fapp.html

### Logs

```bash

```

### System Info

```bash
n/a
```

### Severity

blocking an upgrade