How to keep a window on top of a specific window only?

Question

My application is a Winforms transparent overlay attached to another application.

Since there can be multiple overlays running at one time for different apps, I need my overlay to be always on top, but only over its target application.

What I tried was to bring the form to the foreground with

[DllImport("User32.dll")]
public static extern Int32 SetForegroundWindow(int hWnd);   

When target app gains focus through my WindowHook eventType == NativeMethods.SWEH_Events.EVENT_OBJECT_FOCUS .

The problem is that the overlay gains focus after going on top, not allowing me to interact with the app underneath. Like if I try to drag the target app it will block me at first, and if I try to click anywhere nothing happens despite being clickthrough (works when I set Winforms TopMost = true; ).

I would consider setting the form topmost when target app gains focus, but there is no event to check for loss of focus to undo the topmost, and even then it would be less than an elegant solution, to say the least.

There must be a simpler way to keep my overlay on top of its target app process. Or the very least, a way to bring the form on top without gaining focus / disrupting my mouse events.

Answer 1

So EVENT_SYSTEM_CAPTUREEND = 0x0009 gets called after any interaction with the target process window. This means I can set my form topmost

SetWindowPos(Handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE); and undo it with SetWindowPos(Handle, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);on EVENT_SYSTEM_CAPTUREEND . I'm not a fan of this solution but works and it's seamless.