Like you on this dev page see, when you scroll down, the header disapears. And when you then click on a button, after the header are disapears, to open the offcanvas, the offcanvas are shown, but the header also: https://bildung.fpress.host/
Hi Christian,
Welcome to the forum!
The offcanvas is located in the header, which is transformed out of the viewport. It needs to come back to make the offcanvas visible. You can avoid this by placing the offcanvas element somewhere else (e.g. in the footer or using a template hook to place it above or below the header).
Best regards,
timmse
This make sense 
Works great, many thx