Bug 235116

Summary:	Summing up all movementX of mousemove events does not give the total difference in position
Product:	WebKit	Reporter:	Guillaume <guillaume.brunerie>
Component:	UI Events	Assignee:	Nobody <webkit-unassigned>
Status:	NEW ---
Severity:	Normal	CC:	graouts, thorton, webkit-bug-importer, wenson_hsieh
Priority:	P2	Keywords:	InRadar
Version:	Safari 15
Hardware:	Unspecified
OS:	Unspecified

Description Guillaume 2022-01-12 08:56:41 PST

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).

Comment 1 Radar WebKit Bug Importer 2022-01-19 08:57:15 PST

<rdar://problem/87773443>

Comment 2 Wenson Hsieh 2022-01-21 17:46:58 PST

`WebPageProxy::handleMouseEvent` drops unhandled platform mousemove events on macOS if the mouse event queue is too backed up…

Maybe that is what's causing this behavior?