1846299 - WPT failure in css/css-text/tab-size/tab-min-rendered-width-1.html

Status: RESOLVED FIXED

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

Description

[Thomas Wisniewski [:twisniewski]]

We are failing this test as the 0.7625ch tab size isn't rendering as expected. WebKit 175 recently aligned with Chrome on this test, so it would be good to find out what's going on here.

Comment 1

[Emilio Cobos Álvarez (:emilio)]

Jonathan you added this test and it seems to pass in automation, mind taking a look?

Comment 2

[Emilio Cobos Álvarez (:emilio)]

Test passes locally on Linux here. Hopefully just a test bug?

Comment 3

[Jonathan Kew [:jfkthame]]

The failure is on the line where the initial span on the line has width: 7.5ch, so then the effective width of the tab should be exactly 0.5ch and it shouldn't need to skip to the next tab-stop. But depending on the font in use, the computation of widths in ch units may involve floating-point inaccuracies, and this is potentially making it look as though the tab width ends up fractionally less than 0.5ch.

To avoid this, I guess we could make the testcase use a known font like Ahem, with a size where the ch unit will compute to an exact number of pixels and floating-point width computations shouldn't cause an issue.

Comment 4

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1846299 - Make tab-min-rendered-width test use 16px Ahem as the block font, to avoid floating-point inaccuracies in tab-width computation. r=#layout-reviewers

Comment 5

[Jonathan Kew [:jfkthame]]

This should make the test less fragile with respect to floating-point ch-width computations.

Comment 6

[Daniel Holbert [:dholbert]]

I can reproduce the failure locally, and I confirmed the patch fixes it for me. \o/

(Interesting that it doesn't reproduce on TreeHerder; font-specific stuff I guess.)

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/fda13e44fbdc
Make tab-min-rendered-width test use 16px Ahem as the block font, to avoid floating-point inaccuracies in tab-width computation. r=dholbert

Comment 8

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/41606 for changes under testing/web-platform/tests

Comment 9

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/fda13e44fbdc

Comment 10

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot