Parallax
Header appearance and initial state.
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.