Progress! No more crashing!
Well, at least as long as a window (Tracker in this image) doesn’t get moved onto MiniBrowser.
Technical details
This time, it was just a one line fix. WebProcess tried decoding an integer from a message. Normally, it would just decode the integer directly. But Haiku’s port specified that the integers should be treated specially, as a message attachment. Well, we don’t use message attachments, and, as you can guess, this mistake eventually led to a crash.
The fix: Haiku doesn’t use attachments atm, so just make the attachment type the closest thing to nothing: an empty struct. Now no more integers will be confused as attachments!