Function zone_at 

pub fn zone_at(rect: [f32; 4], pointer: [f32; 2]) -> Option<DropZone>

Classifies a pointer position inside rect into a drop zone.

The centre keeps the majority of the area on purpose: joining a tab group is the common intent, and a split is the deliberate one. Bands are measured against the shorter side so a long thin panel doesn’t turn almost entirely into edge.