[Webkit-unassigned] [Bug 150012] New: Scroll Snap - Viewport jumps to top while another element is transitioning

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=150012

            Bug ID: 150012
           Summary: Scroll Snap - Viewport jumps to top while another
                    element is transitioning
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Macintosh
                OS: Mac OS X 10.11
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bbratvanov at gmail.com

Created attachment 262853
  --> https://bugs.webkit.org/attachment.cgi?id=262853&action=review
Illustrating the scroll-snap bug. Hover on the white bar, then scroll down.

If an element is in the middle of a CSS transition while scrolling and the element's transition duration is longer than the time it takes to scroll and reach the snap point, the viewport will abruptly jump to the very top.

Width, height and other complex transitions manage to trigger the bug every time, but it doesn't seem to happen on simple transitions such as colour changes.

The bug can be replicated in stable Safari 9.0 as well as the latest nightly (r190840 - 601.1.56).

Attached is a test case reduction. It's also available here: https://jsbin.com/qoqiwisuqu

To replicate the bug, just hover on the white bar, wait for the transition of the green span (expanding from 20% width to 100% width) and then scroll down while the span is transitioning back to its initial state.

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