Gizmo click detection is off by a constant

[MattFerraro]

I have a Gizmo on a page that includes an app bar and a tool bar:

When I click on the X, -X, Z, -Y, buttons, the Gizmo does not respond. It behaves as though it is not receiving mouse clicks.

BUT, when I press the Y button, the camera moves to -Y. After some experimentation I found that the gizmo is receiving click events from the following locations:

So I suspect there's a hardcoded assumption somewhere that the <Canvas> extends to the top of the viewport.

[MattFerraro]

Digging in a bit, the assumption seems to be made on this line

I'll see if I can make a patch!

[MattFerraro]

PR is here: #810