ClientX and ClientY for Pointer Events is wrong type

[AndrewLugg]

On PointerEvents, if you look at clientX or clientY it returns an int type, but the value is a double. So it will throw an error if you try to do anything with it.

Example to put into Dartpad to show the issue:

import 'dart:js_interop';
import 'package:web/web.dart';

void main() {

  window.document.addEventListener(
    'pointerdown',
    ((Event event) {
      final e = event as PointerEvent;
      print('clientX: ${e.clientX}');
    }).toJS,
  );
}

Example error:

Uncaught Error, error: Error: TypeError: 317.375: type 'double' is not a subtype of type 'int'

This was tested with Dart SDK 3.11.1 with web 1.1.1 and also with DartPad using 3.11.0

[srujzs]

Hmm, this feels like a divergence in the browsers versus the spec. The definition for PointerEvent extends MouseEvent:

https://github.com/w3c/webref/blob/071b650e6c97649fadf6de9317b4f7d8ef0ad4a6/ed/idl/pointerevents.idl#L59

and MouseEvent has a long clientX and clientY:

https://github.com/w3c/webref/blob/071b650e6c97649fadf6de9317b4f7d8ef0ad4a6/ed/idl/pointerevents.idl#L11

MouseEvent is also discrepant in this case as MDN claims it should return a double value: https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/clientX#value. Testing it in Chrome, it always returns an int value for clientX for mousedown events. Firefox is slightly different where it seems like it rounds clientX values from both mousedown and pointerdown events to the nearest .5.

So, it feels like despite the spec claiming it should return a long value, both Firefox and Chrome can return doubles.

---

Separately, I've been thinking of moving to a different IDL source as the official spec differs from the implementations quite often. One possible source is Firefox's definitions, which in this case does have a double type for clientX/clientY: https://github.com/mozilla-firefox/firefox/blob/12d718e2a701e51587f59c4f1369af6b712fcbda/dom/webidl/MouseEvent.webidl#L31