It's to stop the offset shift. Pages with scrollbars and pages without have different document widths but the same viewport width.
When you click between the two, the page "shifts" half a scrollbar width if it's centered.
To fix this, Apple made the scrollbar an overlay. That stops the shifting. But then everything on any page with a scrollbar looks off-center.
To fix that, Apple made it disappear when it's not being used.
It's to stop the offset shift. Pages with scrollbars and pages without have different document widths but the same viewport width.
When you click between the two, the page "shifts" half a scrollbar width if it's centered.
To fix this, Apple made the scrollbar an overlay. That stops the shifting. But then everything on any page with a scrollbar looks off-center.
To fix that, Apple made it disappear when it's not being used.