Last time I checked the WPF designer, it used canvas by default - which allows you to put elements exactly where you want by simple drag and drop, like the old times in VB6 or Delphi; but then nothing actually reflows or resizes etc at runtime.
If you choose to use layouts, it can display them, but editing it with a mouse is no longer convenient. It's easier to just drop into XAML and hack on it there. On every project I worked on that used WPF (which is quite a few by now), nobody on the team actually used the UI designer, and everybody would switch the default mode for .xaml files in VS to open the markup directly, rather than the usual split view.
WPF and UWP can deal with it perfectly fine.