Bug 311544 – [WPE][GTK] preventDefault() on pointerdown does not suppress gesture-based compatibility mouse events from touch

• Component: WPE WebKit
• Status: RESOLVED FIXED

On WPE (and likely GTK), calling preventDefault() on a pointerdown event does not suppress compatibility mouse events generated from touch input.
After every touch tap, mouse emulation events (pointerId=1, pointerType=mouse) are fired at the same coordinates regardless of whether pointerdown was cancelled.

Per the W3C Pointer Events spec (https://w3c.github.io/pointerevents/#compatibility-mapping-with-mouse-events):
"Authors can prevent the production of certain compatibility mouse events by canceling the pointerdown event."
"Mouse events can only be prevented when the pointer is down. Hovering pointers (e.g. a mouse with no buttons pressed) cannot have their mouse events prevented."

- Test case:
document.documentElement.addEventListener("pointerdown", (e) => {
        e.preventDefault();
}, true);

- How to reproduce:
  1. Create a page with a pointerdown handler that calls e.preventDefault()
  2. Load it in WPE browser with touch input
  3. Tap the screen

- Expected Result:
  - pointerdown (touch) fires, app calls preventDefault()
  - No compatibility mouse events should follow

- Actual Result:
  - pointerdown (touch) fires, app calls preventDefault()
  - pointermove/pointerover with pointerId=1, pointerType=***mouse*** fires ~1ms later

Pull request: https://github.com/WebKit/WebKit/pull/62097

[EWS]

Committed 310827@main (45f6219e113d): <https://commits.webkit.org/310827@main>

Reviewed commits have been landed. Closing PR #62097 and removing active labels.