An in-flight dissolve between two levels of one group. progress ∈ [0, 1)
is the INCOMING level's share of the dissolve; its opacity is
smoothstep(progress) and the outgoing level's the complement. It advances
at 1 / fadeMs per millisecond from startProgress at startMs.
An in-flight dissolve between two levels of one group.
progress∈ [0, 1) is the INCOMING level's share of the dissolve; its opacity issmoothstep(progress)and the outgoing level's the complement. It advances at1 / fadeMsper millisecond fromstartProgressatstartMs.