https://bugs.winehq.org/show_bug.cgi?id=40528
--- Comment #25 from Paul "TBBle" Hampson Paul.Hampson@Pobox.com --- NOTOURBUG makes sense to me. I don't think the Wine Windows API implementation can be reasonably expected to offer random internal windows structures for direct manipulation.
Last time I looked, Cygwin was correctly, but noisily, detecting that it can't find such a structure, which is fine. However, its fallback code was also assuming such a structure existed, and using an even-more-Windows-internal method of trying to get the structure to exist, which happened to crash because it the directly accessed another Windows-internal structure (that Wine doesn't populate) instead of going through the relevant API call.
The warning including "New Windows releases might break this" is a good sign that it's a Cygwin-side thing.