Function fill_measure 

pub fn fill_measure<B: Toolkit>(
    _backend: &mut B,
    _handle: &B::Handle,
    proposal: Proposal,
) -> Size

A leaf measure that FILLS the space it’s proposed — for growing leaves (a web view, a canvas, a Lottie view). Use as measure: day_pieces::fill_measure in renderer!. This gives one uniform “fill” answer across every backend (some backends’ measure: None default returns a view’s natural size, which collapses a size-less native view).