RevisionSlider adds some lines to visually connect the slider with the two revisions on the left and right. For VisualDiffs these are currently hidden, since the diff isn't presented in two columns.
But now that we have the metadata in two columns, the lines by RevisionSlider actually make sense again even in VisualDiff, so I think it's worth considering to add them back (and only change the color). This would also kill an !important from the styles (https://phabricator.wikimedia.org/diffusion/EVED/browse/master/modules/ve-mw/init/styles/ve.init.mw.DiffPage.less;972a1832cc54a208f44f8bd7e564a131b92a2676$36), which always feels good.