1573711 - "text-decoration-skip-ink" doesn't interact properly with "vertical-align"

Status: VERIFIED FIXED

(Core :: Layout: Text and Fonts, defect, P3)

Description

[Daniel Holbert [:dholbert]]

STR:

1. Load attached testcase, with about:config pref layout.css.text-decoration-skip-ink.enabled = true

EXPECTED RESULTS:
The underline should only skip characters that are actively intersecting it.

ACTUAL RESULTS:
For the middle section (the section whose vertical-align is being animated), the underline seems to skip characters as if they had the default vertical-align value.

Note: Chrome isn't useful as a reference case here, because it ties the underline to the characters themselves rather than to the baseline of the element that adds the underline. So it exhibits a different set of ACTUAL RESULTS, though its results do have reasonable ink-skipping behavior.

I'm tentatively marking this as blocking bug 1573631 though I'm not sure it needs to.

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: testcase 1

Comment 2

[Daniel Holbert [:dholbert]]

This affects <sup> tags, for example, which use vertical-align: super via this html.css UA-stylesheet-rule:
https://searchfox.org/mozilla-central/rev/3366c3d24f1c3818df37ec0818833bf085e41a53/layout/style/res/html.css#549-550

Comment 3

[Daniel Holbert [:dholbert]]

Attachment: testcase 2, with partially-transparent super/subscript

Comment 4

[Daniel Holbert [:dholbert]]

Attachment: screenshot of testcase 2 (with some full-page-zoom to make it easier to see)

In this screenshot, you can see that the superscript (2nd word) has unnecessary gaps, whereas the subscript (3rd word) has the line-decoration overlapping the partially-transparent text (not skipping as it should).

Comment 5

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1573711 - Account for baseline offset (potential effect of vertical-align property) when computing skip-ink intercepts. r=dholbert

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/e99be10fe186
Account for baseline offset (potential effect of vertical-align property) when computing skip-ink intercepts. r=dholbert

Comment 7

[Dorel Luca [:dluca]]

https://hg.mozilla.org/mozilla-central/rev/e99be10fe186

Comment 8

[Jonathan Kew [:jfkthame]]

Comment on attachment 9089592 [details]
Bug 1573711 - Account for baseline offset (potential effect of vertical-align property) when computing skip-ink intercepts. r=dholbert

Beta/Release Uplift Approval Request

• User impact if declined: When text-decoration-skip-ink is enabled, rendering is incorrect when vertical-align is used to shift the text baseline (e.g. superscripts/subscripts)
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Simple patch to pass the relevant baseline offset down to the text-decoration rendering code; no impact on anything except how the ink-skipping positions are computed. Includes automated tests.
• String changes made/needed:

Comment 9

[Liz Henry (:lizzard) (relman/hg->git project)]

Comment on attachment 9089592 [details]
Bug 1573711 - Account for baseline offset (potential effect of vertical-align property) when computing skip-ink intercepts. r=dholbert

Fix for new css features which are aiming to ship in 70.
OK for uplift for beta 5.

Comment 10

[Narcis Beleuzu [:NarcisB]]

https://hg.mozilla.org/releases/mozilla-beta/rev/1a8df1c35a02

Comment 11

[Hani Yacoub, Desktop QA]

Verified as fixed on Firefox Nightly 71.0a1 (2019-09-09) and on Firefox 70.0b5 on Windows 10 x64, Ubuntu 18.04 and Mac OS X 10.15.