That package is buggy and deprecated and there seems to be no alternative among WinUI. Andrews solution did not work for me either, it seems they withdrew that package, one cannot find it on nuget (not with the suggested feeds for alternative sources either!) and I think a pre-released/experimental package is not really what a developer who is also serious about his product looking for. So what I did I went with WpfUI, I highly recommend that, GridSplitter is just a native control there, so no need to struggle with preleased and incomplete packages. You won't lack any functionalities with this. https://wpfui.lepo.co/