I added an image to the Horizontal Arrangment, and all of a sudden, it collapsed to a single line. I cannot add anything to it or remove that image from it. I can move it around, and it works in testing. Seems like a bug or something.
I tried to set it to fit parent. but that only works horizontally not vertically.
Ok I figured it out.
I had the image set to 35%, and that caused the problem. So this is a repeatable bug.
Set an image percentage (I had it at 35%), both height and width. Drag it into a horizontal arrangement as the first object. It will collapse to a line that nothing can be added to or taken away.