How did the Windows 95 user interface code get to the Windows NT code base?

Summary

A long time ago, somebody asked, “How did the new Windows 95 user interface get brought to the Windows NT code base? The development of Windows 95 lined up with the endgame of Windows NT 3.1, so how did the finished Windows 95 code get brought to the Windows NT code base for Windows NT 4.0? Did they reimplement it based on the existing specification? Was the code simply merged into the Windows NT code base?” Members of the Windows 95 user interface team met regularly with members of the Windows NT user interface team to keep them aware of what was going on and even get their input on some ideas that the Windows 95 team were considering. The Windows NT user interface team were focused on shipping Windows NT, but they appreciated being kept in the loop. During the late phases of the development of Windows 95, the Windows NT side of the house took a more active role in bringing the Windows 95 user interface to Windows NT. They started implementing the new window manager features that Windows 95 introduced, both in terms of new functions such as Register­Class­Ex and Set­Scroll­Info, as well as new behaviors such as having a Close button in the upper right corner. The window managers on Windows NT and Windows 95 both had ancestry in the Windows 3.1 window manager, so a lot of the designs were the same, but the code had long since diverged significantly, so it wasn’t so much merging the code as it was using the Windows 95 code as a reference implementation when reimplementing the features on Windows NT. (For example, the algorithm for walking the dialog tree in the face of WS_EX_CONTROL­PARENT remained the same, but the expression of the algorithm was different.) The code for Explorer and other user-model components had an easier time. They were taken as-is into the Windows NT code base, warts and all, and the copy was then updated in-place to be more Windows NT-like. For example, the Windows 95 shell used CHAR as its base character, but Windows NT was Unicode-based, so ...