Windows: 1. GWL_USERDATA must be GWLP_USERDATA to be compatible with x86 and x64. (GWL_USERDATA  is deprecated anyway). 3. Replace Get/SetWindowLong with Get/SetWindoeLongPtr or placing this into GWLP_USERDATA will truncate the pointer.
(transplanted from 5f50745bff03700d3862a6bb1eb5936be0fdc6cd)