What happens here is that the arrow position shifts and the old shadow sticks around. The shadow is only computed when the panel is initially shown, or if it is resized. If this is not a recent regression, I would not treat it as overly important, because the Proton theme eliminates the arrow.
Bug 1699435 Comment 1 Edit History
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What happens here is that the arrow position shifts and the old shadow sticks around. The shadow shape is only computed when the panel is initially shown, or if it is resized. If this is not a recent regression, I would not treat it as overly important, because the Proton theme eliminates the arrow.