Horizontal slides editor – How improve its navigation?

I'm building an editor to create a structure of a page based on a full-screen vertical and horizontal slider.

A site is composed for vertical sections. Each of those can have 0 or a limited number of horizontal slides.

Now, I don't know how to create a proper interface to allow them to add more horizontal slides on the right side of the default slide by default.

Right now user add sections and slides by dragging from the panel on the right. The one in the middle is the "default slide" that will be visible when scrolling to that section. Then users will be able to navigate right or left depending on the side the other slides are in.

I'm limited to the space on the viewport and right now, I'm using scrollable right and left sides, so when you add more than 2 slides they won't fit in the viewport. So the way you can access them is by scrolling right or left inside that side of the section, but this doesn't seem right...

Any better ideas of how to deal with this issues?

Some that came to my mind:

• Use an arrow on each row of slides. Click to slide right or left.
• Make the whole element draggable. So user can navigate right or left by draging the whole structure and see the end of the row in any side.
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