Using Unity 2018.2.5f1
Our UI artist created a button with a large glow effect that fades alpha out quite far.
This button is also integrated with the 9-slice feature so it can properly scale horizontally.
I created a sprite atlas and added our UI elements including that particular button.
When I look at the atlas preview, it seems fine. At runtime, that button bleeds a lot and displays some garbage at the bottom. I tried to increase the padding to 8 but it has little to no effect.
After investigation, I came to the conclusion that this might be a bug in the sprite packer. Maybe there is a hidden process that evaluate where to crop the pixels based on an undefined alpha threshold.
→ The issue is that the new height of that image doesn’t match with the height specified in the 9-slice settings of that sprite.