fade
How the strip's alpha follows its target, in both the visible-driven overload and the document-driven scrim. A critically damped spring rather than a tween: the target moves continuously while the page scrolls into the strip, and a spring tracks a moving target smoothly where a tween would restart on every change. The low stiffness makes it settle over roughly a third of a second — slow enough that the strip is never seen to switch, quick enough that a fling has fully covered the strip before the eye gets there. Critically damped so it never overshoots past full or below clear.