|without child horizontal margin||with 20px child horizontal margin|
In the above test cases a box with gray border, light gray background and fixed width (200px) contains a green overflowing child (width: 300px).
When the parent has overflow:auto (or scroll), and the child has horizontal margins then the right margin is not "visible" inside the scrolling area. In other words, most browsers provide just enough scrolling to see the overflowing content, but no more. The overflowing content does not appear to have any 'space' at its right side.
The following test case forces the extra 'scrolling' space at the right of the overflowing content in all browsers. It uses an absolutely positioned element protruding from the overflowing one, of the same width as the right margin (the desired extra space).
This same tests but with overflow in the vertical direction: scrolling (overflow) and children vertical margin.
Similar tests but with padding on the parent, in place of child margin: scrolling (overflow) and padding.
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