Can a window be always on top of just one other window?

Question

In Windows, is it possible to set window A such that it is always on top of window B, yet allow other windows to work as normal and appear over the top of both, when active.

In other words, I want a parent-child relationship between two windows. Can this be done without making window A a child of window B, MDI-style? Window B isn't mine (Internet Explorer), and screws my dialog A's graphics up when I try to achieve this with SetParent.

I thought I'd cracked it with this idea from an MSDN forum post, but alas windows A is still always on top of everything, not just window B.

// Place window A on top
SetWindowPos(hWndWindow, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE|SWP_NOSIZE);
// Place window B underneath it
SetWindowPos(hWndParent, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOSIZE|SWP_NOMOVE);

Is it possible?

Answer 1

Wouldn't creating an ownership relationship do the trick?

SetWindowLong(hwndChild, GWL_HWNDPARENT, hwndOwner)

The windows can be in different processes and you can call this from any process. This will ensure that the child window is always above the owner window. This is different than SetParent which actually creates a Parent / Child relationship. Read through this article (its from 1993 but still mostly correct) to see the distinction between ownership and parenting.

Answer 2

When your window's Z-order (or size or position) is changing, it should receive a WM_WINDOWPOSCHANGING message. If you process that message, you have an opportunity to modify the final Z-order (or size or position) to which the window is moved.

To illustrate, in hWndA's window procedure:

case WM_WINDOWPOSCHANGING:
    DefWindowProc(hWnd, msg, wParam, lParam);
    WINDOWPOS *p = (WINDOWPOS*)lParam;
    p->hwndInsertAfter = hWndB;
    p->flags &= ~SWP_NOZORDER;
    return 0;

should insert hWndA after hWndB in the Z-order any time hWndA's position changes.