Bug 236477 – letter-spacing is totally broken in Burmese and Thai and Lao

• Component: Text
• Status: NEW

[Myles C. Maxfield]

Created attachment 451625 [details]
Test case

WebKit destroys the shapes of the letters. Compare to Chrome / Firefox.

[Myles C. Maxfield]

Created attachment 451626 [details]
Test case

[Myles C. Maxfield]

Oh, wow, ComplexTextController applies letter-spacing to _every glyph_. That's totally bogus.

[Myles C. Maxfield]

It looks like the correct iterator to use is kCFStringCursorMovementCluster.

This is turning out to be harder than I thought it would be. Because Burmese text is so complicated, it looks like I can't:

- add space to the right side of a glyph if the character just after the character the glyph corresponds to is a boundary, because of the string "ဂြို". This string has 4 code points, but no glyph corresponds to the last code point - the last glyph corresponds to the second-to-last and last code points together.
- add space to the left side of a glyph if the character the glyph corresponds to is a boundary, because of the same string "ဂြို". Here, the leftmost character corresponds to string index 1 and then the next glyph corresponds to string index 0. Therefore, if we did this, we'd inject letter-spacing in the middle of the cluster.

I think the solution is to keep track of cluster boundaries as we iterate across glyphs, and insert space to the right of a glyph if the current glyph and the next glyph belong to different clusters.

[Myles C. Maxfield]

https://twitter.com/OhBendy/status/1492033041988001837 indicates that this is a problem for Thai and Lao too.

[Radar WebKit Bug Importer]

<rdar://problem/89118337>