I am trying to create a dialog such that it always opens in the middle of my application window. This is the usual behaviour for a dialog created with something like MessageBox(). However, if the application window is moved before the dialog is opened, the dialog opens in the middle of where the window WAS and not where it is now. So something behind the scenes is obviously caching the position of the window.

Is there anyway to update that position to indicate where the window actually is? The problem is that with MessageBox(), you don’t get options of positioning a dialog, so I can’t easily reposition it correctly.

There’s an interesting answered question here:
How to Change MessageBox Position in C++ and I’ve tried the proposed solution and to be fair, using TaskDialogIndirect() almost works. You can indeed set the (X, Y) position of the dialog, but the X position is slightly out and you can’t alter the dialog size.

So I decided to create the dialog myself and thought that subclassing my main application window was the easiest.

LRESULT CALLBACK DialogWndProc(HWND wnd, UINT uMsg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, [[maybe_unused]] DWORD_PTR dwRefData)
{
    switch (uMsg)
    {
        case WM_CREATE:
            CreateWindowEx(NULL, L"BUTTON", L"Okay", WS_CHILD | BS_DEFPUSHBUTTON | WS_VISIBLE, 50, 50, 100, 40, wnd, (HMENU)1002, GetModuleHandle(NULL), nullptr);
            break;
        case WM_CLOSE:
        {
            HWND owner = GetWindow(wnd, GW_OWNER);
            EnableWindow(owner, true);
            SetFocus(owner);
            DestroyWindow(wnd);
            return 0;
        }
        case WM_NCDESTROY:
            RemoveWindowSubclass(wnd, DialogWndProc, uIdSubclass);
            break;
    }
    return DefSubclassProc(wnd, uMsg, wParam, lParam);
}

// hwd is the HANDLE of the main application window
void CustomDialog(HWND hwd, const std::wstring title, const std::wstring body, size_t xp, size_t yp, size_t xs, size_t ys)
{
    // Get the class name from the parent window
    std::vector<wchar_t> class_name(256);
    class_name.resize(static_cast<size_t>(GetClassName(hwd, class_name.data(), static_cast<int>(class_name.size()))));

    // Subclass the window and change WNDPROC
    BOOL t = SetWindowSubclass(hwd, DialogWndProc, 1, 0);

    // Create the modal dialog window
    HWND dlg = CreateWindowEx(WS_EX_DLGMODALFRAME, class_name.c_str(), title.c_str(), WS_POPUPWINDOW | WS_CAPTION | DS_MODALFRAME | DS_NOIDLEMSG, xp, yp, xs, ys, hwd, NULL, GetModuleHandle(NULL), nullptr);
    ShowWindow(dlg, SW_SHOW);
    EnableWindow(hwd, false);
}

The problem that I’ve found is that WM_CREATE is never received and I don’t understand why. Surely, during the call to CreateWindowEx() the new WNDPROC will receive a WM_CREATE?

Or am I going to have Register an entirely new window class atom for this window?