[Webkit-unassigned] [Bug 260139] New: WebKit doesn't follow UAX14 line breaking rules for EX AL case

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Mon Aug 14 05:11:54 PDT 2023


https://bugs.webkit.org/show_bug.cgi?id=260139

            Bug ID: 260139
           Summary: WebKit doesn't follow UAX14 line breaking rules for EX
                    AL case
           Product: WebKit
           Version: Safari 16
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Text
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: m_kato at ga2.so-net.ne.jp
                CC: mmaxfield at apple.com

This is same as https://bugs.chromium.org/p/chromium/issues/detail?id=1472702. I found this when Gecko is moving to UAX14 for line breaking rules.

Step
====

1. Open data:text/html,<div%20style="width:1px">!ABC</div>

Result
======

"!ABC" is one line

EXPECTED RESULT
===============
"!" is first line, "ABC" is second line.

According to Unicode's UAX14. (https://www.unicode.org/reports/tr14/)
- LB2 Never break at the start of text.
- LB13 Do not break before ‘]’ or ‘!’ or ‘;’ or ‘/’, even after spaces.
- LB31 Break everywhere else.

So "!ABC" string is "EX ÷ AL x AL x AL" as https://www.unicode.org/reports/tr14/#Definitions. But Blink and WebKit seem to be "EX x AL x AL x AL". This behavior is incorrect with UAX14.  

When using 7-bit characters, WebKit uses own line break table (https://searchfox.org/wubkat/rev/cd25edd92284ea5ea247483e66b404c0774949b2/Source/WebCore/rendering/BreakLines.cpp#53). But this rule table isn't compatible with UAX14. So Chrome team should update this table to match with UAX14.

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