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I would probably use a selected state, but then I don't know how important it is for you to keep that kind of animation in the button

Read Stefano's reply more carefully. He did not say use states; he said use the Selected state. Selected is a state with special functions. An object with a Selected state automatically toggles between Selected and Normal when clicked. No triggers needed.

However, at the moment, "...we have an open issue in Storyline 360 where the state change keeps the Normal state's text or image in Preview and Publish." Because your object contains a shape and an oval added to the state, this bug surfaces.

There are still ways to create the same effect. Three of them are included in the attached sample. There is one different method on each slide. Be sure you study and understand all three, so you can make an informed decision about which method you want to use.

Method 1.2 and 1.3 work fine :) Thank you.