[Webkit-unassigned] [Bug 233748] New: Tamil conjuncts are not selected as a single unit when styling initials

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=233748

            Bug ID: 233748
           Summary: Tamil conjuncts are not selected as a single unit when
                    styling initials
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: xfq.free at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 445674

  --> https://bugs.webkit.org/attachment.cgi?id=445674&action=review

Test case

When the start of a line contains a consonant cluster that uses a conjunct (rather than visible virama), ::first-letter should highlight the whole cluster. Usually, modern Tamil has only two of these conjuncts, however one of them can be created in two ways (making a total of 3 clusters to test).

This doesn't work well if segmentation relies on Unicode grapheme clusters, since a conjunct with two consonants will be parsed as two grapheme clusters (the first ending after the virama, and the second starting with the second consonant and including any following vowel-signs or other combining characters).

For these situations it is necessary to tailor the segmentation algorithm, so that it recognises the whole consonant cluster plus any attached vowel-signs or combining characters as a single unit.  This is a particular issue for Tamil, since all other clusters are typically decomposed and show the virama.

Tests & results:

Interactive test, When ::first-letter is applied to Tamil the browser will select the KSHA and SHRI conjuncts as a single unit
https://github.com/w3c/line_paragraph_tests/issues/72

Gecko produces the expected result. Webkit and Blink only select the first consonant+pulli.

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