My simple setup is: - Canvas -- Panel (opaque dark grey) --- Button (unity default button with uisprite with transparent sides, I just darkened the color)
The problem is that the transparent parts of the Button sprite also makes the panel transparent. This makes the panel look like it has a transparent border around the button.
See image. See the light areas around the buttons, this is actually the background being visible through the panel.
I have tried various image import settings. I have tried psd and png format with a square image and only defining transparency with the alpha channel.
It still has the transparency issues with the panel.