Can't set Z order in Horizontal boxes?

I’m trying to set set to be above a progress bar within a horizontal box, however I don’t have the option to set Z Order.

you can set the z order of a horizontal box and all the contents of that box will be at that z order, the horizontal box is to arrange widgets side by side.

is it possible to have a different behaviour ? i’m trying to make a slider but the right button is preventing me to click the bar properly at the right edge