Sets the type of animation used for reveal and fold transitions in this.
See [the migration guide](migrating-to-breakpoints.html#replace-adwflap)
[property@Flap:flap] is transparent by default, which means the content will be seen through it with [enum@Adw.FlapTransitionType.over] transitions; add the [`.background`](style-classes.html#background) style class to it if this is unwanted.
| this |
a flap |
| transition_type |
the new transition type |