I didn't get much farther by myself, but I recently ran across a snippet from someone who seems to know what they're doing. The quote is from Jason Schroeder. In case you're wondering how this fits into my original question, I found a stack trace in a dump file that shows that SendMessageW calls NtUserMessageCall. Here is the quote: user32!NtUserMessageCall is just a stub that calls into win32k.sys, so if you want to see what your thread is "really" doing, hook up a kernel debugger... Reference: http://ask.metafilter.com/75491/Why-is-SendMessage-blocking-on-NtUserMessageCall[^]