Shorthand properties of computed CSSStyleDeclaration don't return correct values

[jdm]

<div id="test" style="overflow: hidden"></div>
<script>onload = function() {
  var div = document.getElementById('test');
  console.log(getComputedStyle(div).overflow)
  console.log(getComputedStyle(div).overflowX);
};
</script>

In Firefox, this shows hidden twice. In Servo this shows an empty string, followed by hidden.

[jdm]

There's a comment in processed_resolved_style_request and process_resolved_style_request_internal that looks fishy to me given Firefox's behaviour:

        // Firefox returns blank strings for the computed value of shorthands,
        // so this should be web-compatible.

[jdm]

@emilio This behaviour was introduced in 0ad2d39. Firefox appears to behave differently now; do you know when that changed?

[jdm]

Mmm, maybe it was introduced in 58d452f instead.

[jdm]

Chrome also returns real values for shorthands, so Servo's behaviour is definitely not web compatible.

[jdm]

I think the right thing to do here is implement the same shorthand logic as Servo_GetPropertyValue in at least process_resolved_style_request_internal, possibly process_resolved_style_request as well.

[jdm]

Ah, I see w3c/csswg-drafts#2529 and the note in the spec for step 5 of https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle which explicitly mentions overflow.

[emilio]

Sorry I never replied here. Shorthands are special, but firefox only serializes some (in particular, it only serializes the ones that used to be longhands).