1891446 - Enable symmetrical CSS letter-spacing on Nightly

Status: RESOLVED FIXED

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

Description

[Jonathan Kew [:jfkthame]]

To help us understand whether the behavior (and spec) change proposed in https://github.com/w3c/csswg-drafts/issues/10193 is actually feasible, I'd like to try enabling the symmetrical letter-spacing behavior on Nightly to see how it fares.

With the option that was implemented in bug 1890773, this is a simple pref change. It will affect a bunch of reftests, because letter-spaced text will be shifted by ½-letter-spacing in the inline direction; this is the expected result. So eventually if we do agree to deploy this change, we'll want to update expectations for the relevant tests.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1891446 - Experimentally enable symmetrical letter-spacing in Nightly builds.

This implements the behavior proposed in https://github.com/w3c/csswg-drafts/issues/10193.
Pending discussion of the issue there, and as input to that eventual debate, I'd like to
try enabling this behavior on Nightly to see how it goes in the wild.

This results in a bunch of failures on existing reftests, in an entirely predictable way,
because any text with letter-spacing gets offset slightly, and no longer matches with
references that assumed glyphs remain flush-left in their advance. If we do adopt the
symmetrical model across the web platform, corresponding test adjustments will be needed.

(We also get a few new passes, on tests involving letter-spacing + bidi, which is the case
where our legacy behavior is particularly poor, and the symmetrical model resolves the
issues.)

For the time being, with the pref change only on Nightly, I propose to just add pref
overrides in the manifests for the affected tests so that they stay green for both Nightly
and non-Nightly builds.

Comment 2

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1891446 - Adjust prefs in test manifests to keep letter-spacing tests green for now. r=#layout

Comment 3

[Jonathan Kew [:jfkthame]]

For reference, here is a tryserver run that has the pref change to enable the new behavior, but without touching any test metadata. Reviewing the failures here, we can see that they are all caused by the expected shifting of letter-spaced glyphs to the middle of their modified advance widths.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/7acc4ef7877f
Experimentally enable symmetrical letter-spacing in Nightly builds. r=layout-reviewers,emilio
https://hg.mozilla.org/integration/autoland/rev/8d4ab18c9e32
Adjust prefs in test manifests to keep letter-spacing tests green for now. r=layout-reviewers,emilio

Comment 5

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/7acc4ef7877f
https://hg.mozilla.org/mozilla-central/rev/8d4ab18c9e32

Comment 6

[Dipika]

Note for MDN writers: letter-spacing docs are not impacted, but worth a mention in release notes

Comment 7

[Dipika]

This has been added to the list of Experimental features in Firefox on MDN (the PR was just merged so the update will be available in a day).
The doc update can be tracked via https://github.com/mdn/content/issues/33521.