In prosilver, there's a scrollbar at the right bottom corner of attachbox which is displayed all the time regardless of overflow.
Disabling the overflow-x here https://github.com/phpbb/phpbb/blob/8a8403ff8aeec8319da803a656b7051c96a0dd2d/phpBB/styles/prosilver/theme/content.css#L555 solves the issue.