not sure what your main interest is here but that seems to only work while the mouse pointer is within the form.
For anything outside the form client's area it would be the Non-Client so a WM_NCRBUTTONUP..
However, there are some areas with the LCL that blocks what is considered non useable messages for a TFORM. Its not like Delphi where all messages are pipped through and finish with the default procedure.
You can however, hook the window procedure to capture all of it.