Libertinus: Add stylistic set to disable Th ligature or move it to a different ligature set

[alerque]

As recently noted on Twitter some users (and some other people) have a personal beef with the Th ligature. Some also argue it should be a decorative ligature instead of a standard one.

I don't actually have a dog in this race, but it seems like either a way to disable this (perhaps a stylistic alternate that explicitly does not include it) or moving it to an optional decorative ligature set would be in order.

See also this issue on EB Garamond.

[KrasnayaPloshchad]

ZWNJ could be used to disable this.

[dajare]

Thanks for taking this on board. As mentioned in my Twitter post, the solution in Linux Libertine Graphite was to add lith switch, so the font setting Linux Libertine G:lith=1 would prevent the ligature:

I suppose the advantage of that approach is that it does not require moving this ligature to another "set" -- as I understand it, this switch simply provides a specific over-ride. But I may not understand this quite correctly!

One disadavantage is that it creates a non-standard feature switch, although I don't know how serious this consideration is.

For the time being, inserting U+200C is a viable work around, although not ideal.

[khaledhosny]

U+200C is the proper Unicode way to prevent a ligature. But since I don’t like Th ligatures in general, I’m open to having a feature to disable it. I’m not going to use a non-standard tag though since this won’t work on applications that do not allow using feature tags directly. Using a stylistic set, ssXX, is a good compromise.

[KrasnayaPloshchad]

Thanks for taking this on board. As mentioned in my Twitter post, the solution in Linux Libertine Graphite was to add lith switch, so the font setting Linux Libertine G:lith=1 would prevent the ligature:

Linux Libertine G is used Graphite layout to produce this feature, so such feature switch is available only in Graphite aware applications.

[KrasnayaPloshchad]

I think Th ligature can be moved to another ligature rules, but still available with liga feature: T[ZWJ]h.

[khaledhosny]

I opted to moving Th ligature to Discretionary Ligatures (dlig) feature, but also it can be enabled by explicitly inserting ZWJ between the two letters.