Linux app with Header Bar (GTKLib)

I’m working on a Gnome (GTK) Linux app at the moment, and am looking to add a GTK Header Bar and Buttons within it, as per standard Gnome applications, eg: Text Editor (see below).

I’ve found GTKLib by @Ulrich_Bogun which seems to have the basics and seems like it should do what I need, but the header bar buttons seem to be full height by default, as below. I found the BorderWidth property which lowers the height but makes the spacing between buttons too large.

Screenshot - GTKLib

Any advice anyone can provide would be great to make the header buttons ‘normal’ that would be very appreciated!

Adding an example with BorderWidth = 8 for context/further example.

Screenshot GTK-Lib BorderWidth

In case anyone has similar issues in the future, the issue was dragging the GTKLib into a brand new project created with Xojo 2021 R3, which changed the GTKLibWindow super to DesktopWindow instead of Window as per the original/example project. With the example project, where the super is Window all works as expected.