I see in the documentation that the window has a property headHeight: http://docs.webix.com/api__ui.window_headheight_config.html, but that doesn’t work. As far as I can see in the code, the headHeight property is never read from the config.
You are correct. The problem was that I tried adding it to the defaults of a custom component and those are not always used: http://webix.com/snippet/476f1a5b
I’ve fixed this by copying the default values to the config in my custom $init method.
Thanks for the help.