No idea. I’ve never needed a collaborative whiteboard, and for non-collaborative use I haven’t found anything on the web anywhere near as good as the desktop options of Microsoft Whiteboard and Krita (though Krita has a definite learning curve), so on the odd occasions I want to share things with others I just do screen sharing of these. I started making one of my own a few years back and had the basic interactions down pat, but shelved that project before it ever got really good or could have supported collaboration. Mine was also bitmap-based rather than vector-based, which is a significant difference in approach in some regards, losing you some functionality but gaining you a lot of speed.