ShowOn Y value ignored if more than 1 instance

If I load on map more than one control-window, they can't be aligned on the same raw.
Even if I use the showOn method setting the same Y value for each, only the first one has the defined Y value, others are set below it, according to the X value.

It appears the control-window can not be nested with others on the same row whatever the method I use (show(), show(position) or showOn.