How can I prevent my EditorGUI.Maskfield from showing Mixed... ?

I am using a maskfield in my custom editor;

selectedValue = EditorGUI.MaskField( position,, selectedValue, options.ToArray() );

When multiple values are selected, it is displayed like this;

Unity’s layermask looks like this, with multiple values aligned horizontally

(changing the width of the field does not yield any different behavior)
Any idea how I could mimic this behavior?

You could draw a label on top with same style:
GUI.Label(rSummary, summaryLabel, EditorStyles.popup);

Just generate a proper summary string.