Confused about Widget Mouse Capture

I’m confused about how Mouse Capture in widgets works.
I have a widget with a Canvas Panel, Sizebox, and a couple of Buttons that I’m trying to use as a modal Dialog box, so I don’t want mouse input going to anywhere else when it’s open.
I’m using the following blueprint in the OnMouseMove event to do this. But while it does capture mouse input, the Buttons become un-responsive. It seems that the mouse events are only going to the top-level widget and not getting passed to its sub components. What am I missing?