Window "on desktop"

I was trying to do the same... i've used a lot of ideas arround, but i was able to to it and prevent the flickering.

I managed to override WndProc, used one setwindowpos before to put it in the background, and another to prevent it from getting the focus...

    const UInt32 SWP_NOSIZE = 0x0001;
    const UInt32 SWP_NOMOVE = 0x0002;
    const UInt32 SWP_NOACTIVATE = 0x0010;
    const UInt32 SWP_NOZORDER = 0x0004;
    const int WM_ACTIVATEAPP = 0x001C;
    const int WM_ACTIVATE = 0x0006;
    const int WM_SETFOCUS = 0x0007;
    static readonly IntPtr HWND_BOTTOM = new IntPtr(1);
    const int WM_WINDOWPOSCHANGING = 0x0046;

    [DllImport("user32.dll")]
    static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X,
       int Y, int cx, int cy, uint uFlags);
    [DllImport("user32.dll")]
    static extern IntPtr DeferWindowPos(IntPtr hWinPosInfo, IntPtr hWnd,
       IntPtr hWndInsertAfter, int x, int y, int cx, int cy, uint uFlags);
    [DllImport("user32.dll")]
    static extern IntPtr BeginDeferWindowPos(int nNumWindows);
    [DllImport("user32.dll")]
    static extern bool EndDeferWindowPos(IntPtr hWinPosInfo);

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        IntPtr hWnd = new WindowInteropHelper(this).Handle;
        SetWindowPos(hWnd, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);

        IntPtr windowHandle = (new WindowInteropHelper(this)).Handle;
        HwndSource src = HwndSource.FromHwnd(windowHandle);
        src.AddHook(new HwndSourceHook(WndProc));
    }

    private IntPtr WndProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        if (msg == WM_SETFOCUS)
        {
            IntPtr hWnd = new WindowInteropHelper(this).Handle;
            SetWindowPos(hWnd, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);
            handled = true;
        }
        return IntPtr.Zero;
    }

    private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
    {
        IntPtr windowHandle = (new WindowInteropHelper(this)).Handle;
        HwndSource src = HwndSource.FromHwnd(windowHandle);
        src.RemoveHook(new HwndSourceHook(this.WndProc));
    }

My answer is in terms of the Win32 API, not specific to WPF (and probably requiring P/Invoke from C#):

Rainlendar has two options:

• "On Desktop", it becomes a child of the Explorer desktop window ("Program Manager"). You could achieve this with the SetParent API.
• "On Bottom" is what you describe - its windows stay at the bottom of the Z-order, just in front of the desktop. It's easy enough to put them there to begin with (see SetWindowPos) - the trick is to stop them coming to the front when clicked. I would suggest handling the WM_WINDOWPOSCHANGING message.

This is what I used so the window is always "on bottom":

   using System;
    using System.Runtime.InteropServices;
    using System.Windows;
    using System.Windows.Interop;

...

[DllImport("user32.dll")]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X,
   int Y, int cx, int cy, uint uFlags);

const UInt32 SWP_NOSIZE = 0x0001;
const UInt32 SWP_NOMOVE = 0x0002;
const UInt32 SWP_NOACTIVATE = 0x0010;

static readonly IntPtr HWND_BOTTOM = new IntPtr(1);

public static void SetBottom(Window window)
{
    IntPtr hWnd = new WindowInteropHelper(window).Handle;
    SetWindowPos(hWnd, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);
}