[webkit-changes] [WebKit/WebKit] b0183a: starlink.com: no deceleration scrolling (momentum)...

[Simon Fraser]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: b0183a11772926cc7a259d4f220ec9e5015a94fa
      https://github.com/WebKit/WebKit/commit/b0183a11772926cc7a259d4f220ec9e5015a94fa
  Author: Simon Fraser <simon.fraser at apple.com>
  Date:   2025-01-28 (Tue, 28 Jan 2025)

  Changed paths:
    M Source/WebCore/page/scrolling/ScrollingTreeScrollingNode.h
    M Source/WebCore/page/scrolling/mac/ScrollingTreeOverflowScrollingNodeMac.mm
    M Source/WebCore/page/scrolling/mac/ScrollingTreePluginScrollingNodeMac.mm

  Log Message:
  -----------
  starlink.com: no deceleration scrolling (momentum) on pages with non-root `background-attachment: fixed` in overflow scroll
https://bugs.webkit.org/show_bug.cgi?id=286604
rdar://129302721

Reviewed by Matt Woodrow.

During the momentum phase of a two-finger trackpad scroll, we synthesize events via
MomentumEventDispatcher. These events, handled by
`RemoteLayerTreeEventDispatcher::handleSyntheticWheelEvent()`, never go to the web
process, because that function doesn't call `continueEventHandlingOnMainThread()`.

This is a problem when `ScrollingTreeOverflowScrollingNodeMac::handleWheelEvent()` returns
early if the node has synchronous scrolling reasons (of which "has slow repaint objects"
is one); the synthetic event neither triggers UI-side scrolling, nor goes to the web
process to scroll.

Fix by having two non-root `handleWheelEvent()` functions not early return if the only
synchronous scrolling reason is for non-fast repaint. We will correctly repaint when
the web process is notified about the scroll.

Not testable, because synthesized wheel events don't contain the data necessary for
MomentumEventDispatcher to work.

* Source/WebCore/page/scrolling/ScrollingTreeScrollingNode.h:
* Source/WebCore/page/scrolling/mac/ScrollingTreeOverflowScrollingNodeMac.mm:
(WebCore::ScrollingTreeOverflowScrollingNodeMac::handleWheelEvent):
* Source/WebCore/page/scrolling/mac/ScrollingTreePluginScrollingNodeMac.mm:
(WebCore::ScrollingTreePluginScrollingNodeMac::handleWheelEvent):

Canonical link: https://commits.webkit.org/289465@main



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