[Webkit-unassigned] [Bug 235116] New: Summing up all movementX of mousemove events does not give the total difference in position

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=235116

            Bug ID: 235116
           Summary: Summing up all movementX of mousemove events does not
                    give the total difference in position
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: UI Events
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: guillaume.brunerie at gmail.com

The specification of the MouseEvent.movementX/Y properties says that

> All motion data must be delivered via mousemove events such that between any two mouse events earlierEvent and currentEvent the value of currentEvent.screenX-earlierEvent.screenX is equivalent to the sum of all movementX movementY/code> events after earlierEvent, with the exception of when screenX can not be updated because the pointer is clipped by the user agent screen boundaries.
(see https://w3c.github.io/pointerlock/#dom-mouseevent-movementx)

It works perfectly fine in Chrome but does not seem to work as expected in Safari, as soon as I move the mouse I get differences of several hundreds of pixels.
See the following CodePen: https://codepen.io/Latcarf/pen/YzrRvZE

Tested with Safari Version 15.1 (15612.2.9.1.30, 15612), on MacOS Catalina version 10.15.7 (19H1615).

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