Zero-width non-joiner behaves incorrectly with some ligatures

Hiya,

I really don't know much about fonts and font development in general, so please feel free to close this issue if the described behavior is intended or likely caused by something else :)

I noticed some weird behavior with U+200C (Zero-width non-joiner). To my understanding, inserting this character should prevent the surrounding characters from forming a ligature together. In Fira Code, this behavior can be seen with the != ligature:

• the sequence != displays as a ligature
• the sequence !\U200C= does not display as a ligature

However, for some ligatures, this does not work. The following sequences seem to consistently render as ligatures, even though they shouldn't:

• -\U200C> renders the -> ligature
• ~\U200C> renders the ~> ligature
• -\U200C- renders the -- ligature

And likely more. All of this was tested both in Kitty and in IntelliJ IDEA, with Fira Code 6.2.

Even more weirdly, when disabling ligatures (at least in IntelliJ, in Kitty disabling ligatures seems to just disable ligature rendering alltogether as U+200D doesn't even do anything anymore) the insertion of U+200C seems to force these ligatures to be rendered even if they otherwise wouldn't be, behaving the same as U+200D!

I tested this in the software that was available to me, if there's a more direct way to reproduce this without potentially being biased by application-specific ligature rendering weirdness do let me know. Again, I know very little about font developent.

Thank you! :)

Reproduce the reported sequences with Fira Code 6.2 in Kitty or IntelliJ IDEA, comparing U+200C and U+200D with ligatures enabled and disabled. Determine whether the behavior is in the font or the rendering application; done means the affected sequences consistently follow the expected zero-width non-joiner behavior, with a direct reproduction path documented.