Bug 179237 – Automatic linebreaking of WebVTT subtitles is not supported

• Component: Media
• Status: NEW

[Frédéric Wang (:fredw)]

From the spec: https://w3c.github.io/webvtt/#introduction-multiple-lines

"Line breaks in cues are honored. User agents will also insert extra line breaks if necessary to fit the cue in the cue’s width. In general, therefore, authors are encouraged to write cues all on one line except when a line break is definitely necessary."

This page uses the example from the specification:

https://people.igalia.com/fwang/webvtt-tests/subtitle-linebreaking.html

Linebreaking happens for the long (third) subtitle in Gecko, but not in WebKit. I tried both WebKitGTK+/Linux and Safari/macOS.

[Frédéric Wang (:fredw)]

Created attachment 326204 [details]
WIP Patch

I had reported this to Chromium and they reply it is fixed (see https://bugs.chromium.org/p/chromium/issues/detail?id=781186). The attached patch is based on https://chromium.googlesource.com/chromium/src.git/+/e6b16d301f0da8a22bfba7ab5997771af0459cf5%5E!/#F2 and removes a feature that is no longer in the WebVTT spec. It turns out to fix the linebreaking issue.

I also opened bug 179370 to import the WebVTT tests.

[Ahmad Saleem]

All browsers are failing WPT test linked in the URL (removed the standard link URL since it is in Comment 0). Added 'WPTImpact' tag for future tracking.

[Ahmad Saleem]

We still have this code but in different shape here:

https://searchfox.org/wubkat/source/Source/WebCore/html/track/VTTCue.cpp#261

if (!cue->snapToLines()) {
        setInlineStyleProperty(CSSPropertyWhiteSpaceCollapse, CSSValuePreserve);
        setInlineStyleProperty(CSSPropertyTextWrapMode, CSSValueNowrap);
}