I've had this weird problem, in that my MDI child proc was not recognising the WM_CREATE message, although Spyxx showed the message passing through this window proc ???.
I traced it down to the WM_CREATE section, setting MsgBox windows as an indication that the execution had arrived at that point.
This whole dam week, I've been trying different angles around this proc with Zip, Nudder, no result.
I cannot tell what I've done exactly after all this 'dancing around'.. but now it's working after going through the same set of code combinations many times.
Has anybody had this problem, or is it peculiar to the latest Masm64 'compiler'.
It has driven me nutz this week

Oh.. I created the MDI Child window with the CreateWindowEx Function.
mTYPE_proc proc hWin:DWORD, uMsg:DWORD, wParam:DWORD, lParam:DWORD
; mov eax, uMsg
; RegVal eax
.if uMsg == WM_CREATE
mov eax, 10810
RegVal eax
Invoke mTYPE_InitMem, Memory_Heap.h_MainHeap, c_MEMALLOC_8MB
RegVal eax
Invoke mTYPE_CreateControls,hWin
RegVal eax
.elseif uMsg == WM_MDICREATE
; mov eax, 10820
; RegVal eax
.endif
invoke DefMDIChildProc,hWin,uMsg,wParam,lParam
ret
mTYPE_proc endp