Configure view size to allow animation to start at center of view.

[jscottsmith]

Sometimes users want the animation to begin at the middle of the screen instead of at the bottom. Or they want the animation to end early before the element leaves the view. See react-scroll-parallax #157

To allow this an option can be added to scale the view size (the portion of the scroll area where the element animates).

The new option viewportSize should accept pixels or percent values via the box model. Percent would be based off the view size.

viewportSize: [top, right, bottom, left]

Examples

To configure an element to start animating at the middle of the screen the bottom of the view is adjusted to half the view size:

viewportSize: [0, 0, 50%, 0]

To configure an element to end animating at the middle of the screen the top of the view is adjusted to half the view size:

viewportSize: [50%, 0, 0, 0]

[grandma228]

Fully support!