Bug 240509 – Incorrect line breaking for Javanese

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Norbert Lindenberg]

https://lindenbergsoftware.com/apple/safari/javanese-lb.html
contains a <p> element with style="overflow-wrap: break-word".

According to https://drafts.csswg.org/css-text/#overflow-wrap-property, this enables line breaks "at an arbitrary point if there are no otherwise-acceptable break points in the line", under the constraint that "grapheme clusters must stay together as one unit". (With 

https://drafts.csswg.org/css-text/#character makes clear that "A UA must use the extended grapheme cluster (not legacy grapheme cluster), as defined in UAX29, as the basis for its typographic character unit" but may tailor it where necessary to improve behavior.

The actual line breaking for Javanese with this setting shows several issues, that can be seen by resizing the window and thus the paragraph.

Screenshot 1 shows that a dependent vowel, U+A9B4 ◌ꦴ, is separated from the other characters of the extended grapheme cluster ꦪꦺꦴ it belongs to.

Screenshot 2 shows that an extended grapheme cluster ꦪꦺꦴ is not separated from the preceding extended grapheme cluster ꦤ꧀ꦲꦁ even though it clearly does not fit on the line.

Screenshot 3 shows that similarly several other extended grapheme clusters are allowed to stay on lines that they don't fit into: ꦧ꧀, ꦤ꧀ꦤꦺ, ꦤ꧀.

[Norbert Lindenberg]

Created attachment 459483 [details]
Screenshot 1

[Norbert Lindenberg]

Created attachment 459484 [details]
Screenshot 2

[Norbert Lindenberg]

Created attachment 459485 [details]
Screenshot 3

[Radar WebKit Bug Importer]

<rdar://problem/93810908>

[Norbert Lindenberg]

These line breaking problems are no longer reproducible in Safari 18.3 on macOS 14.7.3. I assume that’s because WebKit relies on ICU for line breaking: The Unicode Line Breaking Algorithm was updated in Unicode 15.1 to support line breaking at orthographic syllable boundaries, and ICU implemented that change.

See:
– Proposal: https://www.unicode.org/L2/L2022/22080r2-line-break-ortho-bnd.pdf
– UAX 14 in Unicode 5.1: https://www.unicode.org/reports/tr14/tr14-51.html
– ICU 74 release notes: https://icu.unicode.org/download/74