I'm trying to trap mouse events in WPF by using a topmost, transparent non-modal window. I'm finding that this works fine if the opacity of the window is 0.01 or greater and it has a background color, but when the opacity is set to 0 it no longer receives mouse messages. Is there a way make this window look fully transparent and still get mouse input?
As far as I know, no.
When a Control or Window is fully transparent, it can then be clicked through. This is the case if you set your Window.Background="Transparent", or Opacity="0". As far as I know, this is by design in WPF.
When using an opacity of 0.01, you should barely see the window, if at all. This is likely your best bet at achieving the functionality.
Edit: Another solution, which I tried and does work, is to set the background color to an almost-transparent color. I used Background="#01000000", thus giving an alpha value of 1. This makes your window background transparent-looking, but allows you to place controls on it with the window at full opacity.