PROMPT_SEARCH_REWINDOW_MARGIN = 5How close to a mounted edge the selection gets before the window re-centers.
update_selection re-styles two rows when the selection stays comfortably
inside the mounted window, and rebuilds all of it otherwise. Rebuilding
whenever the ideal start moved would rebuild on every single-step move once
the list outgrows PROMPT_SEARCH_WINDOW, which is the common case. This margin
keeps that on the cheap path until the selection nears an edge.