Hopefully I can describe this effectively: For a recent project I made a stacked bar chart using pure CSS and percentage units. Each section of the bar has a percentage based height. All the sections heights add up to 100%. However, unless the container height is such that the percentage based heights map to an exact integer pixel size (e.g. 9% of 500px is 45px, but 9% of 380px is 34.199px), the total height of the boxes is consistently less than the total height of the container. I made a demo: http://apps.npr.org/lookatthis/posts/publichousing-webkit-bug-demo/ (actual in-the-wild project here: http://apps.npr.org/lookatthis/posts/publichousing) The percent heights add up to 100%, but the boxes themselves occupy a few pixels less than 100%. The same behavior can be seen using viewport-relative units like vh. I was able to replicate in Safari (OS X, iOS) and Chrome (on OS X, Android, iOS). It doesn't show up in Firefox.
It looks like percent sizes do not get subpixel layout. There is a block of 380px split in 33%, 33%, 34%. The computed size of each block is 125px, 125px, 129px. It should ideally be 125.4, 124.4px, 129.2px. Zalan would know why we don't get subpixel layout in that case.
Created attachment 243775 [details] Test reduction.
I am not able to reproduce this bug using attached test reduction on STP173 and it matches Chrome Canary 117. @Alan - I think we can close this, appreciate if you can confirm.