I have this shape made up of lines (see left side of image). Why is it that when I resize it to a smaller size, it becomes slightly transparent (see right side of image)?
Changing it to a smart object has no effect.
Duplicating the resized one multiple times makes it appear opaque (as it should), but merging all the duplicated layers results in the transparent version.
How can I resize it to smaller without this weird transparency?
(I’m trying to make a UI kit where I can copy/paste/resize elements without their look changing. I would like to avoid Illustrator if possible.)
This is exactly why you want to use Illustrator as you can keep the strokes a certain width even as you scale. If you are serious about UI kit, I would put in the extra time to learn Illustrator.
Your resizing shrinks everything along with the thickness of the lines. Your shape is not transparent, it just has really thin lines. You will need to deal with this in vector shape if you want to avoid that.
You may avoid the shrinking effect in Photoshop without using vectors if you use a stroke on a layer effect(outer border only) or a graphic styles but that offers fairly limited options.
If you want to thicken a shape like that, you could select it the strokes only, expand the selection by 1 pixel and fill it with black although it sounds like terribly bad practice to me, it might just do the thing for what you want to accomplish.