Why the slots(child) in uniform grid panel dont have the same size?

So i have my inventory widget which have : Sizebox->Border->ScrollBox->UniformGridPanel to store my slot widgets, as for the slot i have : Sizebox->CanvasPanel and the CanvasPanel has Text and Border1->Border2->Button->Image then i set Border2 to have a padding of 1 so the slot seems to have an outline. Why are my slots outline behave this way? Does anyone have experience with this kind of stuff and want to share some knowledge?

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