WM_GETTEXT retrieving partial output

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C++

Win32

WinAPI

I'm trying to pick only the pasted part of a text inside a subclassed editbox while intercepting the WM_PASTE case, in the following way :

case WM_PASTE:
	{
                //i'll later need this two integers to pick only pasted text
		int length = SendMessageW(MainEdit, WM_GETTEXTLENGTH, 0, 0);
		CallWindowProc(DefaultEditProc, hwnd, uMsg, wParam, lParam);
		int length2 = SendMessageW(MainEdit, WM_GETTEXTLENGTH, 0, 0);

		LPSTR buffer = (LPSTR)GlobalAlloc(GPTR, length2 + 1);

		GetWindowTextA(MainEdit, buffer, sizeof(buffer));
                //send message displaying the same wrong output
		//SendMessage(MainEdit, WM_GETTEXT, sizeof(buffer), (LPARAM)buffer);

		MessageBoxA(hwnd, buffer, "nnt", MB_OK);

		GlobalFree(buffer);
    }
    break;

The code above shows in the messagebox only 3 characters out of supposed 16 (or whatever number, for all that matter)

Tried also using a normal buffer :

wchar_t buffer[] = new wchar_t[length2 + 1];
memset(buffer, 0, sizeof(buffer));
//getwindowtext or sendmessage retrieving the same wrong output
GetWindowTextA(MainEdit, buffer, sizeof(buffer));
//GetLastError() always returning 0
delete[] buffer;

If I were to use a normal array buffer the code runs just fine, but since my project involves undefined-lenght pasting processes I can't just place a fixed value.