Of course you don’t need to have a Binding for the color of the vertical line – you could hard-code that in your template, especially if you are only going to have one of them.
The default behavior is to put them all in one row and then wrap them to fit in the viewport. This is implemented by the Palette.layout, which is a GridLayout.
But if you don’t mind there maybe being a horizontal scrollbar, then you can set GridLayout.wrappingWidth to a large number, so that it won’t (ever?) need to wrap. When initializing the Palette, perhaps: