A step read the day the layer had last published, which reaches the
shortcut and the header arrows through useSearchParams, a render and an
effect. Next 16.4 lets that chain trail a replaceState by more than a key
repeat, so two quick presses of ] both stepped from the same day and one
was lost. stepDay now reads ?day= from window.location, which the step
itself writes, and refuses a step past today on that value too.