Function paint_order_of 

pub fn paint_order_of(
    e: Entity,
    _parents: &HashMap<Entity, Entity>,
    cache: &mut HashMap<Entity, u32>,
) -> PaintOrder

Returns the PaintOrder for e - the document-order rank precomputed by build_parent_map (whose second return value is the map passed here as cache).

Entities absent from the map (standalone drawables with no hierarchy links) fall back to 0x8000_0000 | (entity_index << 1): they paint after all tree content in entity-allocation order, and - because the high bit clears every tree rank - no ExtractedClipBox descendant range can accidentally absorb them.