Slider always has default width in table

The folution is, that it's inside an table so the width is defined by the table width attibut for the spec. col and row.

So the fix is pretty short:

table.add(volumenSlider).width(600).height(60);

And its 600width and 60 height.

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The wiki got edited to be more clear about this:

UI widgets do not set their own size and position. Instead, the parent widget sets the size and position of each child. Widgets provide a minimum, preferred, and maximum size that the parent can use as hints. Some parent widgets, such as Table, can be given constraints on how to size and position the children. To give a widget a specific size in a layout, the widget's minimum, preferred, and maximum size are left alone and size constraints are set in the parent.

Layout at Wiki