To make a fixed menu like this:

1. First you have to switch to Advanced mode

2. Select the Navigation Widget in Widget Tree

and in Widget Properties Panel/Advanced/Position select FIXED

3. In Widget Properties Panel/Advanced/Z-index type in 3. The z-index property specifies the stack order of an element.

An element with greater stack order is always in front of an element with a lower stack order.

Note: z-index only works on positioned elements (position:absolute, position:relative, or position:fixed).