1401870 - Mark is positioned incorrectly after forming ligature

Status: NEW

(Core :: Graphics: Text, defect, P3)

Description

[Norbert Lindenberg]

Attachment: HTML document testing with the font

The attached font and HTML file demonstrate a bug that was found during development of a Burmese font. Burmese requires glyph reordering, ligature formation, and mark-to-base and mark-to-mark positioning.

In the test font, the marks u (U+102F) and anusvara (U+1036) are positioned relative to the base nna (U+100F) via anchor attachments. When tested by themselves, this works fine. Positioning of the anusvara also works fine when a medial ra (U+103C) is present. However, when a medial ra is present, the font forms a ligature from medial ra and u in order to position them together. The rule for this is

feature blws {
    script mym2;

    lookup raMedial_uMark_myanmar {
        lookupflag IgnoreBaseGlyphs UseMarkFilteringSet @below_base_myanmar;
        sub raMedial-myanmar uMark-myanmar by raMedial_uMark-myanmar;
    } raMedial_uMark_myanmar;
};

After the ligature is formed, the anusvara is no longer positioned correctly. It appears that the shaping engine loses track of nna as the base and positions relative to the medial ra + u ligature instead.

The same problem does not occur in Safari 11.0.

Firefox 55.0.3

Comment 1

[Norbert Lindenberg]

Attachment: Glyphs source for the test font

Comment 2

[Norbert Lindenberg]

Attachment: Test font

Comment 3

[Norbert Lindenberg]

Attachment: Rendering on Firefox 55.0.3

Comment 4

[Norbert Lindenberg]

Attachment: Rendering on Safari 11.0

Comment 5

[Jonathan Kew [:jfkthame]]

This would be a harfbuzz issue, better reported at https://github.com/behdad/harfbuzz/issues. (Note that Chrome renders the same as Firefox.)

Also, how does it render in MS Edge?

Comment 6

[Norbert Lindenberg]

Filed as harfbuzz issue:
https://github.com/behdad/harfbuzz/issues/543

Comment 7

[Firefox Bug Husbandry Bot]

https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Move_fix-optionals