Such helpful answers here. Were you able to resolve your fluid boxes?

Fluid boxes are in a nested structure. In the parent fluid box which covers the whole slide you can have a number of vertical OR horizontal fluid boxes. Due to the way fluid boxes are used to make the structure fit in any size of the browser window, portrait or landscape you cannot have both in the parent fluid box. Within the child fluid boxes you can again choose between vertical and horizontal. The properties of the fluid boxes are important, so it is possible to have a column of fluid boxes switch to a row when the screen requires that.

I don’t understand how you see a mix of vertical and horizontal in one fluid box? Could you post a mockup (image) of such a situation, maybe we can explain how to realize it?

You must decide the first which one is ok for your design. You can draw your design on a paper (sometimes I do)

For example, you can add 3 vertical rows and you can add 2 horizontal columns in the second row which was created first. I mean, you can add horizontal to a vertical row or you can a vertical row to a horizontal column.