[abenedik]

By default, the FitToHeight method will start an animation that will slowly change the Viewbox to the viewbox that will show the content in the available height.

This is also written in the help file and online help for the FitToHeight method - see https://www.ab4d.com/help/ZoomPanel/html/M_Ab2d_Controls_ZoomPanel_FitToHeight.htm

So, you just need to set IsAnimated to false before calling FitToHeight. Then the Viewbox will have the final value immediately after calling FitToHeigth.