https://bugs.winehq.org/show_bug.cgi?id=24501
--- Comment #19 from Vincent Povirk madewokherd@gmail.com --- I've started work on importing this. I figure we'll deal with any missing parts as we go. For now, System.Xaml is in on the master branch, and WindowsBase builds but its System.Windows.Threading implementation doesn't work.
It's still a low priority for me and likely to take a long time regardless.