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What is this blue outline box?
Is this blue box a new guide to visually tell me two objects on the same layer? Or does it do something else? What do I do with it or how do I trun it off temporarily?

Is this blue box a new guide to visually tell me two objects on the same layer? Or does it do something else? What do I do with it or how do I trun it off temporarily?

Without knowing more about your setup, my guess is you have Layer Edges on. You can turn this setting on and off from View > Show > Layer Edges.
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