Jetpack Compose: Update composable when list changes

You need to use a MutableStateList<T> so that Compose can automatically recompose when the state changes.

From official doc:

Caution: Using mutable objects such as ArrayList<T> or mutableListOf() as state in Compose will cause your users to see incorrect or stale data in your app.

In your code use

val favourites = remember { mutableStateListOf<Track>()}

instead of

var favourites: MutableList<Track> by mutableStateOf(mutableListOf())