Sorry, you can close this – not a bug.
There is an onclick event attached to the BECanvas (attached using JSInterop) that fires a C# method that closes the modal window.  When resizing the window smaller the onclick event on the BECanvas is firing when I was not expecting it to as I’m not clicking on it.  I’ve attached OnResizeStart and OnResizeEnd events to the Window object to switch a flag on and off that the BECanvas onclick event checks (to ensure window is not being resized) before closing the modal window.