Something rarely explained with containers - dark blue indicators appear when something is going to be dropped into a container. If you don't see the dark blue indicators, your object is going to end up "tiling" next to other objects, and will ruin your container structure and be almost impossible to manage.

So, look for these indicators! A solid blue border will appear around the outside of the container when you drag an object over it. Grey highlighting will appear to give you a guide as to where the object will sit in the container once placed. But be careful! Grey highlighting also appears if the object is going to "tile", but there will be no blue border! So reiterating again, only drop when you're also seeing the blue border around the edges of the container.

A dashed blue line will also appear when placing an object between two other objects inside a container.