PostMessage() succeeds but my message processing code never receives the message

In my C++ application's GUI object I have the following in the main window procedure:

case WM_SIZE:
    {
        OutputDebugString(L"WM_SIZE received.\n");
        RECT rect = {0};
        GetWindowRect(hwnd, &rect);
        if (!PostMessage(0, GUI_MSG_SIZECHANGED, w, MAKELONG(rect.bottom - rect.top, rect.right - rect.left))) {
            OutputDebugString(L"PostMessage failed.\n"); // <--- never called
        }
    }

    return 0; // break;

The GUI object also has the following getMessage() method:

int GUI::getMessage(MSG & msg) {
    BOOL result = 0;

    while ((result = GetMessage(&msg, 0, 0, 0)) > 0) {
        if (msg.message > (GUI_MSG_BASE-1) && msg.message < (GUI_MSG_LAST+1)) {
            OutputDebugString(L"GUI message received.\n");
            break;
        }
        else {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return result;
}

The application object calls this method in the following way:

while ((result = _gui.getMessage(msg)) > 0) {
    switch (msg.message) {
        // TODO: Add gui message handlers
        case GUI_MSG_SIZECHANGED:
            OutputDebugString(L"GUI_MSG_SIZECHANGED received.\n");
            _cfg.setWndWidth(HIWORD(msg.lParam));
            _cfg.setWndHeight(LOWORD(msg.lParam));
            if (msg.wParam == SIZE_MAXIMIZED)
                _cfg.setWndShow(SW_MAXIMIZE);
            else if (msg.wParam == SIZE_MINIMIZED)
                _cfg.setWndShow(SW_MINIMIZE);
            else if (msg.wParam == SIZE_RESTORED)
                _cfg.setWndShow(SW_SHOWNORMAL);
            break;
    }
}

The application object is interested in the window size because it stores this information in a configuration file.

When I run this in Visual Studio's debugger, the output window looks like this after resizing the window:

W