There seems to be no way to map a window without focusing it. I've only tested this on X11, but it may be present on other platforms as well given there is no WindowBuilder option to disable the behaviour.

On X11, this code is the culprit:

XMapRaised pushes the window to the top of the stack as well as maps the window.

There should be an option to disable this behaviour and map the window normally with XMapWindow().