Unify the code paths for smooth scrolling, and scrolling to snap points

• Component: Scrolling
• Status: RESOLVED CONFIGURATION CHANGED

[Simon Fraser (smfr)]

Smooth scrolling uses ScrollAnimator's ScrollAnimationSmooth. Snap point animations use ScrollingMomentumCalculator stuff. They should be the same.

[Simon Fraser (smfr)]

We have way too many scroll animators.

[Simon Fraser (smfr)]

Maybe bug 218857 addresses this.

[Martin Robinson]

I've done a quick inventory of all of the scroll animation classes and here's a summary:

ScrollAnimator: WebProcess-only class providing operations that can scroll from one point to another and manages scroll snap / rubber banding via ScollingEffectsController. This is only used by ScrollableArea. I think it exists to avoid a layering violation, since it depends on platform-specific behavior.
  * ScrollAnimatorMac uses NSScrollAnimationHelper (private API) to scroll WebProcess-side scroll areas with animations.
  * ScrollAnimationSmooth: Generic implementation of smooth scrolling animation. This is used by ScrollAnimator for operations like "scroll to point with animation" on non-Mac platforms. On Mac this is used for CSSOM smooth scrolling (fixed by https://bugs.webkit.org/show_bug.cgi?id=218857). In addition, it's used in the UIProcess by the Nicosia scrolling tree.

ScrollingEffectsCoordinator: Manages rubber banding and scroll snapping in both the WebProcess and UIProcess (scrolling tree).
  * ScrollSnapAnimatorState: Manages state for scroll snapping. Only used by ScrollingEffectsCoordinator.
    * ScrollingMomentumCalculator - Abstract base class that calculates movement for animations involving scrolling momentum. ScrollSnapAnimatorState uses this when ScrollingEffectsCoordinater sets up a momentum animation based on kinetic scrolling / scroll snap. Only Mac does this currently.
   * ScrollingMomentumCalculatorMac: Mac implementation of ScrollingMomentumCalculator which uses NSScrollingMomentumCalculatorSPI to calculate the momentum values.
   * BasicScrollingMomentumCalculator: Platform-independent implementation of ScrollingMomentumCalculator. I think this is unused, but could one day be used for kinetic scrolling on other ports.

* ScrollAnimationKinetic: Kinetic scroll animation class that's only used for Nicosia Scrolling Tree kinetic animations. I'm unsure how much of this could be replaced by ScrollingEffectsCoordinator and BasicScrollingMomentumCalculator.

I think there's lots of opportunities for simplification here. In particular it seems like ScrollingEffectsCoordinator / ScrollSnapAnimatorState / ScrollingMomentumCalculator could be collapsed down into a single class and greatly simplified. 

Another thing that could happen is that a lot of ScrollAnimator could move into ScrollableArea and the platform-dependent parts could be implemented as a delegate that only does the actual animation.

These are just some initial ideas, but there's lots of room for improvement. The code is a bit convoluted, because we have to send information down to all of these helpers whenever scroll snap points change.

[Simon Fraser (smfr)]

> a lot of ScrollAnimator could move into ScrollableArea

Maybe, although it would be nice to think of snap points as a "scroll destination modifier", perhaps a ScrollableArea helper but not in the same class. We should see if there's scrolling tree code related to snap points that duplicates logic in ScrollAnimator.

> ScrollAnimatorMac
I intend to remove its m_scrollAnimationHelper and the delegate, and replace it with a ScrollAnimation subclass. After doing that, macOS just uses the m_scrollAnimation in the base class, and maybe ScrollAnimatorMac can mostly go away.

I've love to avoid both ScrollAnimator and ScrollController having platform subclasses/implementations. That's a source of a lot of brain hurt.

> ScrollAnimationKinetic
See https://bugs.webkit.org/show_bug.cgi?id=230385. I want to add a macOS specific ScrollAnimationKinetic variant that uses ScrollingMomentumCalculator. Maybe ScrollAnimationKinetic can just use BasicScrollingMomentumCalculator (or move the BasicScrollingMomentumCalculator code into ScrollAnimationKinetic).

[Radar WebKit Bug Importer]

<rdar://problem/83407810>

[Simon Fraser (smfr)]

These are both ScrollAnimation subclasses now. I think we can call this done.