Outliner rows are exactly UI_UNIT_Y high and are shown in alternating darker/lighter colors.
When rows are highlighted - for selection, hover, or search - the highlighted bar does not take up the entire vertical space but instead leaves a gap. That gap generally looks like a dark line under the highlighted row.
The dark line looks like a separator between the items above (and including) the selection and the items below. It also adds complexity and fussiness for no real benefit.
It also takes space away from the row when all the space is needed since active icons have a background circle that takes up almost the entire vertical space. So this dark gap removes the small margin below that icon.
The following image shows how it looks now on the top, showing the gap below the blue bar. The bottom half shows how it looks after this patch is applied with the gap removed, and the icon margin restored.
This change is based on the positive response to the things proposed here.