When the overflow of an element is set to scroll, then a scroll bar is always visible on the element whether it is needed or not (it appears as inactive but it is still visible).
The primary reason behind this is if your design is intended to be pixel accurate.
If you care about pixel accurate measurements, then the last thing you want is the bar being removed and you having to recalculate positions. When scroll is used you know the bar will always take up that much space even if it’s not being used.
When it is set to auto, then the scroll bar appears only if it is needed (for example if element’s inner content height is more than element’s height).
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