[Webkit-unassigned] [Bug 145148] New: Scroll-snap animations should not start on axes with zero-delta

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=145148

            Bug ID: 145148
           Summary: Scroll-snap animations should not start on axes with
                    zero-delta
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bfulgham at webkit.org

I noticed a bug in two-dimensional scroll snap regions as follows:

1. Begin a vertical scroll-snap gesture.
2. Scroll region being a vertical scroll animation.
3. Interrupt the vertical scroll with a horizontal gesture.
4. A horizontal scroll will begin.
5. Suddenly, the horizontal scroll will end and a vertical scroll will start.
6. The remainder of the original vertical scroll completes.

This is happening because of a flaw in the gesture handling code.

a. Vertical gestures are tracked, and a vertical target is selected.
b. Horizontal gesture interrupts animation, clearing the wheel tracking state.
c. Horizontal gesture is tracked, causing non-zero X delta values to be tracked, as well as an equivalent set of zero Y delta values.
d. When the horizontal gesture enters the inertial scrolling phase, WebKit activates the scroll snap logic.
e. WebKit first checks for vertical scrolling. It sees that it has a valid count of wheel events, and triggers the start of a vertical scroll snap animation.
f. The vertical scroll animation begins overriding wheel events.
g. Later, WebKit checks for horizontal scrolling. However, it sees that wheel events are being overridden, and stops.
h. Consequently, the horizontal snap is "converted" to a vertical snap.

The fix is to block starting the scroll snap animation on an axis if the user gesture involved zero delta in the direction of the axis.

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