I have modified the standard info window to include self-defined tab-bar at the top.
Hence the auto-panning isn’t working correctly, as there is a part of the InfoWindow that isn’t recognized as such resulting in this flawed output:

Now I want to offset the panTo-position that gets calculated by infoWindow.open() for ~30px on the y-axis, yet I do not know if it’s possible, and if so, how.

There’s no offset directly related to the area used by the infoWindow when calling
open()method. You should setdisableAutoPantotrueon infoWindowOptions object and usepanTo()map’s method with an offset on theLatLngobject passed.