Function paint_tile 

pub fn paint_tile(
    out: &mut OutputTile<'_>,
    layout: &Layout,
    bounds: Rect,
    source: &[u8],
    source_width: usize,
    source_height: usize,
    bg_rows: &[u32],
    bg_columns: &[u32],
    ignore: &[Region],
)

Chrome/content synthesis (buildFramedCanvas’s pass 1) for one output tile: every pixel of bounds (already clipped to the framed canvas) either copies a native reference pixel, paints a background band, or is left untouched (reconstructed-content interior, filled later by fold_evidence).