changing property contentsGravity in transform-only layer, will have no effect

I started to create a very simple tic-tac-toe game. The main goal is to make the view proportional to all screen sizes of all iOS devices. So I put the image on a ViewController, make it full size of screen and then I put it into a Stack View. I've added constrains to this Stack View 0 to all sides. And when I ran the simulator then everything looks good. But I receive a message in console panel. What does it mean?

It could happen because you changed the Mode attribute of the StackView:

I got this warning when I set Mode to Aspect Fit.

Any layer change on a UIStackView will result in this warning. As the warning points out UIStackView is a transform-only layer and it differs quite slightly from a UIView. As per documentation:

The UIStackView is a nonrendering subclass of UIView; that is, it does not provide any user interface of its own. Instead, it just manages the position and size of its arranged views. As a result, some properties (like backgroundColor) have no effect on the stack view. Similarly, you cannot override layerClass, drawRect:, or drawLayer:inContext:.

More on this here.

It may not point to the exact source of the OP's issue but hopefully, it will shed some light on why UIStackView behaviour is not the same and should not be confused with UIView.

For someone that need: I set clipToBounds to false in StackView.