| Is there any tutorial for User32.dll with descriptions of the commands?
User32.dll powers the entire Windows interface, so... no, not really. Certainly not from a Blitz perspective; using it kinda goes around what the Blitz tools are supposed to help you with.
The only good way to address this will be to study Windows UI programming in general and then apply that knowledge to BP (not least because if you don't understand these commands in their intended context, you will have a much worse time using them in a weird environment like Blitz userlibs). I suggest starting here: http://msdn.microsoft.com/en-us/library/windows/desktop/ff657751%28v=vs.85%29.aspx
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