Primarily, this is because i want to educate myself how stuff is working.
I bind the Model in the XAML of CustomDiagram, and also set HasDefaultModel to false, otherwise Links from the LinksSource are not shown/rendered. - This however also leads to, that tools which i use define in XAML directly are not used. - I think this is a timing issue of GoWPF.
Anyway, i create all my custom tools within my CustomDiagrams constructor. - With that, it seems the tools are the ones that are actually used. - Only with that way, i hit breakpoints on some overwritten stuff.
I was able to create the BoxTemplate within the CustomDragSelectionTool’s constructor:
So for me this is solved now. - Still find it strange, that the Diagram behaves strange when used with MvvM binding mechanism’s. - There seems to be some kind of order someone has to follow to get the diagram working as expected.