Are extremely tight leggings appropriate for work?

Hello! I'm trans, although I am a trans man (I was assigned female at birth and now identify as male), rather than a trans woman, but I may be able to assist. At the end of the day, your coworker is, in fact, a woman, and is not doing anything different to the other women in your office. If she was requested, by HR or otherwise, to stop wearing tight clothing, then she may have a case for discrimination under the protected characteristic of gender reassignment (I hate the term but that's what the law uses). In the case that you wanted her to stop, then the official dress code as outlined by HR would have to be changed for all women in the office. Socially, I genuinely cannot predict how she may react if you decide to ask her to change her office dress. She seems to be fairly relaxed and public about her identity, so she may react well. She might also be upset about it and go to HR, which opens a whole new can of worms. Trans people are not a monolith and I have no clue how she might react. To answer the question in the title, if other women in the office are wearing tight leggings, then I'd say that a trans woman has a right to wear them too, even if it displays a difference in genitalia. At the end of the day, it's just a penis. Half of the population has one and it's no use getting yourself into a tizzy over it.