Function overwrite_cost 

pub fn overwrite_cost(
    line: &[Cell],
    style: &Style,
    from: u16,
    to: u16,
) -> Option<usize>

Approximate byte cost of re-emitting the cells in line[from..to] as a forward-move candidate.

Returns None when any width > 0 cell in the range has a style or link that does not match the active pen — in that case the row cannot be re-emitted without an interleaved pen change and the candidate is not eligible.

The cost approximation is the sum of cell.width() over occupied cells in the range. For plain ASCII this equals the emitted byte length exactly. For wide CJK glyphs (width == 2, multi-byte content) and other multi-byte single-width glyphs (combining sequences, emoji selectors) the prediction underestimates the emitted length, so an overwrite candidate may be picked here that a strict byte minimisation would have rejected. Accepting this approximation keeps the predictor allocation-free.