Parallax
Parameters
How much of the scroll distance the header content moves by. 0f pins it, 1f scrolls it with the body, 0.5f is the classic parallax.
When a drag or fling ends with the header partly collapsed, settle it to a resting position instead of leaving it there. A fling settles in its direction; a plain release settles by snapThreshold.
Collapse progress, 0f..1f, at or past which a plain release settles collapsed rather than expanded. 0.5f settles to the nearer position; 0.75f favors expanded. In ScrollMode.EnterAlwaysCollapsed the same threshold applies to the toolbar's exit.
How the header and toolbar react to the body scrolling; see ScrollMode.
Fade the whole header out as it collapses. Turn off, together with a parallaxMultiplier of 0f, when header elements use the per-element modifiers of ParallaxToolbarScope instead.
Let a downward drag past the top stretch the header, which zooms its content and pushes the body down, then springs back on release. Pair with the layout's onStretchTrigger for pull-to-refresh.
How far the header must be stretched when released for onStretchTrigger to fire.
Animation used when the header settles on its own: snaps, stretch releases, and collapse()/expand() when they are not given a spec. Use a snap() or a short tween to honor a reduced-motion setting.