Function selection_changed 

pub fn selection_changed(
    old: Option<&Value>,
    new: &Value,
    select: &[String],
) -> bool

True when any selected field differs between two versions of a document.

Drives event suppression on projected subscriptions: a subscriber that asked for four fields has no way to observe a write that touched none of them, so waking it costs a full client-side rebuild for nothing.