Almost certain to be wrong or incomplete, this somewhat demonstrates the kind of behavior of interest.

Deleted: liga.diff 1.5 KB

liga.diff 1.5 KB Download

Thanks for the report. Was this an open source font that we could use for testing?

I noticed this behavior while testing the behavior of SansBullshitSans which is hosted on github and Apache 2.0 licensed (it's Droid Sans with three extra glyphs and a bunch of long ligatures). I think the version on github has enough ligatures of the appropriate sort to see this issue, I'm currently testing with a slightly altered version with a few more ligatures.

Attaching a test case. This is SansBullshitSans but with the 'liga' feature not marked as required. By default ("font-variant-ligatures: normal;") the ligatures without spaces are applied and the ligatures with spaces are not applied. If "font-variant-ligatures: common-ligatures;" is added to the text area then the ligatures with spaces start showing up. It doesn't appear that "font-variant-ligatures: common-ligatures;" is actually changing any font features (compared to 'normal'), instead it appears to be simply toggling the word by word shaping. The behavior seems to be the same with #enable-layout-ng on and off.

Deleted: index.html 305 bytes

index.html 305 bytes View Download

Deleted: SansBullshitSans_LigaNotRequired.ttf 51.9 KB

SansBullshitSans_LigaNotRequired.ttf 51.9 KB Download

Playing around with the above example shows that #enable-layout-ng is improving things (apparently by not using the word cache). The above example works as described, but only because the text is in a textarea (for ease of changing the size). With #enable-layout-ng off the same behavior is seen if the textarea is changed to a div, with #enable-layout-ng on the behavior when the textarea is changed to a div is improved with the ligatures being used as expected. It appears this is because layout-ng doesn't do the per-word shaping, but textareas aren't yet layout-ng-ified.

It appears that ComputeCanShapeWordByWord is the only user of kLigatures, so it seems that after layout-ng lands it can simply be removed. It appears there is no user of kCaps currently? Also FontDescription::SetDefaultTypesettingFeatures doesn't appear to be used. kKerning does appear to be used, but the default is always off? If so, it looks like UpdateTypesettingFeatures is evaporating?

Just discovered that this is covered by wpt test font-features-across-space-3.html which layout-ng has marked as Pass. However, this test doesn't cover the case of this happening in a textarea and layout-ng still doesn't cover that, so I'm hesitant to mark this as fixed by layout-ng yet.