When you hover your cursor over a new building block, you should be able to see it appear as a "ghost" (grayed out, maybe flashing slightly) to get an idea of the size, shape, and how it will look in that container.
When you hover your cursor over a new building block, you should be able to see it appear as a "ghost" (grayed out, maybe flashing slightly) to get an idea of the size, shape, and how it will look in that container.