1700152 - WPT test css/css-font-loading/fontface-override-descriptors.html fails although the descriptors are implemented

Status: NEW

(Core :: Layout: Text and Fonts, defect)

Description

[Jonathan Kew [:jfkthame]]

With the patches in bug 1681691, these tests would be expected to pass, and indeed they do on Android. But on desktop platforms we're seeing a slight baseline discrepancy between tests and reference renderings. So we should figure out where that comes from and if we can resolve it.

Comment 1

[Jonathan Kew [:jfkthame]]

Turns out these tests also pass on desktop Linux (which isn't too surprising, as the two freetype backends are fairly similar in how they handle font metrics etc), so the failures are just on macOS and Windows. Probably related to rounding or similar issues in those font backends.

Comment 2

[Jonathan Kew [:jfkthame]]

OK, I was a bit confused among several testcases; now that the dust has settled on bug 1681691, it seems that just the one testcase fontface-override-descriptors.html is failing, across all platforms.

It looks like the override descriptors are largely working as expected, but we're ending up with slightly different baseline alignment of the Ahem glyphs than the reference file expects. So this is probably an issue in how we use the metrics to compute the final line height and frame ascent used by layout.