Function merged_row_width 

pub fn merged_row_width() -> f64

The room a header row keeps at its trailing end for the window’s furniture ([RowFurniture]): the window’s own controls, the window’s search and its new-tab door, each with the gap the bar keeps between its sections. Every window that lets its row hold the bar reserves this, so its app’s own controls end where the window’s begin.

The furniture laid out in a row wider than any window says it: what a row keeps and what stands in it are one arrangement, never two that can drift apart.