Function edge_mist_for 

pub fn edge_mist_for(viewport: Rect, target: Point) -> Option<EdgeMist>

Compute the edge-mist cue for target relative to viewport.

Returns None when target is inside (or exactly on the border of) the viewport — an on-screen peer needs a real cursor, not a mist. When the peer is off-screen the dominant overflow axis picks the edge: a peer mostly to the right gets a Right band even if slightly above the top, so the single strongest cue reads unambiguously.