Xaml Package Manager now includes a Xaml Designer, available as a second tab in the editor. It's a little rudimentary (and probably always will be) but it is still useful for some editing. Since there is the possibility of doing more harm than good to an innocent and unsuspecting xaml file if it's used by an unskilled operator, it is currently only available when the logged-in user is named Paul Harrold or Phil Pearson.

You can make changes in the xaml editor and then switch to the designer to see the effect, or vice versa.

There are some limitations in what can be displayed and in the accuracy of some things like font size because the display is not hosted in Welder and therefore doesn't have all of the infrastructure support that Welder provides. Also, when you make changes in the designer and save them back to xaml, each element is written to a single text line. That means that some lines get veeeeeeeeery looooooooooooong. It's a nuisance; it doesn't affect operation but it makes viewing and editing in xaml less easy. You can always put line breaks back in wherever you want them. A xaml formatter is on my todo list, but don't hold your breath!