Bug 271694 – MouseEvent offsetX and offsetY should be relative to padding edge

• Component: UI Events
• Status: RESOLVED FIXED

[Jonathan Heathcote]

Created attachment 470601 [details]
Demo showing mouse offset coordinates from MouseMove on an element with a thick border.

Observed in Epiphany 45.3 on Arch Linux.

MouseEvent events include offsetX and offsetY values which should be relative to the border box of the event target (see spec: https://drafts.csswg.org/cssom-view/#dom-mouseevent-offsetx). In WebKit, however, offsetX/Y coordinates appear to be relative to the border box.

The attached demo page shows the clientX/Y values when the mouse is moved over a div. When moving the mouse over the thick black border, negative values should be shown (see Firefox and Chromium for correct examples) however positive values are shown instead in WebKit.

NB: this appears to be similar (but different) to bug #218117 (MouseEvent.offsetX/Y returns wrong value for inline elements) since this issue relates to block elements and it seems coordinates come relative to the border box, not the padding box.

[Alexey Proskuryakov]

Reproduces in Safari too

[Radar WebKit Bug Importer]

<rdar://problem/125763807>

[Ahmad Saleem]

Our implementation:

https://searchfox.org/wubkat/rev/2f857be7d42f00feb7b59870709310154181a774/Source/WebCore/dom/MouseRelatedEvent.cpp#183

Blink:

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/events/mouse_event.cc;l=452;drc=2827c7359e94610318802bff2cd4e4d8256cd215

[Ahmad Saleem]

Pull request: https://github.com/WebKit/WebKit/pull/56304

[EWS]

Committed 305479@main (2a4ece3e4b02): <https://commits.webkit.org/305479@main>

Reviewed commits have been landed. Closing PR #56304 and removing active labels.