This has caught me out three times now – I build style, but the top border for the global links ‘My Links’ link didn’t show:

I checked my style, and there it was. So I tried adding a margin, or padding in the cell that contained it.

Wrong. It needs it’s height to be set to 100%.

.ms-globalbreadcrumb .ms-HoverCellActive,
.ms-globalbreadcrumb .ms-HoverCellInctive,
.ms-globalbreadcrumb .ms-HoverCellActiveDark{
height:100%;
padding-top:1px;
}