WM_WhatTheHeckIsThis

[jj2007]

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WndProc proc uses esi edi ebx hWnd, uMsg, wParam:WPARAM, lParam:LPARAM
  .if uMsg==0C0BFh
MsgBox 0, str$(uMsg), "Unknown message:", MB_OK
  .endif

Can't find any reference to this message :(

Full source & exe attached, pure Masm32.

P.S: Got it - try invoke RegisterWindowMessage, chr$("TaskbarButtonCreated")

Some windows messages are indeed undocumented, tends to be a crime committed 25+ years ago that they can never fix again. Messages >= 0xc000 are pretty normal, RegisterWindowMessage() produces them. These messages mean something to DefWindowProc or a sub-classed window procedure, they are not intended for your consumption. – Hans Passant Jul 6 '16 at 13:20