How to use window focus messages for a Delphi on-screen keyboard form

I need a built-in on screen numeric keypad in my Application. For various reasons I cannot use the TMS Software or other commercial component offerings. I'm very happy with a button-based solution shown below but I cannot yet see how to solve the focus switch issue where clicking the button activates the keypad form and I lose the focused control into which I wanted the characters. My solution works if I keep the keypad buttons within the target form but I would like a form-independent solution. Is there a way of disabling the button activation or knowing where the focus came from so that I can use something like Scree.ActiveControl :=?? to put it back?