As I was working on a Linux C# application, I was kinda annoyed with the new dialog not having keyboard control. The window would actually display correctly on top of its owner, but it would never take the keyboard focus onto itself. Thus, no keyboard control. A bit of searching also showed that this was already a known issue, sitting pretty for a while now.
But thankfully, the workaround is rather simple. From within your code, just attach (or override) to your Activated event and manually select the button.
Activated += delegate { button.Focus(); };With this, Avalonia will switch focus to the new window, and everything else will work as expected.