Function opacity_model 

pub fn opacity_model(
    main: &mut World,
    parents: &HashMap<Entity, Entity>,
) -> OpacityModel

Resolves CSS opacity for the frame.

An element with Opacity below one and a visible child in its own paint band is a group: it and its subtree paint at full alpha into a layer that fades once (ExtractedOpacityGroup). Every other faded element folds its opacity into its own paints. A paint inside a group therefore carries none of that group’s alpha. The one way out of an ancestor’s group is an OverlayLayer root, whose subtree paints in the top-layer band outside the group’s bracket; its paints carry the opacity of every element above it instead. Returns an empty model when no entity carries an Opacity (the overwhelmingly common case, so extracts skip the lookup entirely).