Determines when the flap will fold.
See [the migration guide](migrating-to-breakpoints.html#replace-adwflap)
If set to [enum@Adw.FoldThresholdPolicy.minimum], flap will only fold when the children cannot fit anymore. With [ enum@Adw.FoldThresholdPolicy.natural], it will fold as soon as children don't get their natural size.
This can be useful if you have a long ellipsizing label and want to let it ellipsize instead of immediately folding.