I've been pulling my hair out trying to understand what's going on here.
I have an image which is of a book and its shadow. The book part is opaque, and the shadow part has semi-transparency. This is shown here with a plain white background so you can see it. The background is actually transparent.
This is then made into a Smart Object, and scaled down, and placed on top of a background. Now, out of nowhere, a pixel of transparency appears at the border between the opaque book and its shadow:
To be clear, the book and its shadow are a single raster pixel layer inside the Smart Object; and there is no empty pixel between them!
Anyone have an idea what is going on?