NoBorder - bakkeby/dusk GitHub Wiki
The NoBorder flag indicates that the client window is to be drawn without a border.
In practice the way this works is that if the flag is set then the border width of the client is set to 0 when the window is registered by the window manager.
This also means that the flag only has an effect when set via client rules or when the window manager restarts, so enabling the flag during runtime won't affect the border of the client window.
If you want to change the border size on a per client basis during runtime then refer to the setclientborderpx function.