Implement CGJ-based diacritics.

[dscorbett]

An above-base diacritic following an above-base double diacritic should be rendered above it. For example, here is <U+0075, U+0361, U+034F, U+0301, U+0069> from Unicode’s figure 7-11.

The same goes for below-base diacritics following below-base double diacritics.

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• Your font version: 3.1.1
• Your font variant: Iosevka Extended
• Your operating system (name and version): macOS 10.14.6
• Your application using Iosevka: hb-view

[be5invis]

@dscorbett AFAIK such CGJ-based tie-marks are not supported by any fonts I know. Could you please provide an example of one font supported that?

[be5invis]

This must be the trickiest OTL feature I ever made, but it works. The idea is that if an above mark is after an above tie, then it will be replaced into a mark with the same shape but different mark class, and that attaches to the tie.
Close as fixed.

The string is u\u0303\u0361\u034f\u030ei\u030a u\u030e u\u034f\u030e u\u0303\u035f\u034f\u0348i\u030a u\u0348 u\u034f\u0348.

[dscorbett]

The only other fonts I know that support this are Noto Sans, Noto Sans Mono, and Noto Serif.

They manage it without substituting marks for clones in different mark classes. Can’t you use the same anchor for attaching U+030E to U+0361 as for U+030E to (e.g.) U+0300? The only difference is that U+0361 would have no attaching mark anchor point for that anchor, just a base mark anchor point.