Groups items onto the given days (keyed by DayKey). A spanning item lands on every visible day it covers; each day's items are ordered by start. Work per item is bounded by the visible range, so a long span costs no more than the days on screen.
Groups items onto the given days (keyed by DayKey). A spanning item lands on every visible day it covers; each day's items are ordered by start. Work per item is bounded by the visible range, so a long span costs no more than the days on screen.