HTML <tr> tag and position:relative

I'm also not entirely sure without seeing code, but:

From the spec: http://www.w3.org/TR/CSS21/visuren.html#propdef-position

The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.

tr is a table-row.

The interaction is probably that position:relative is disabling the border-collapse property. Of course, without seeing any code, it's pretty difficult to tell.

Update: If you look at your code you will see that the tr in question never has a border of its own. Setting position: relative affects display on the th elements. I suggest that you put this down to undefined behaviour.

If you need to position the row relatively, then I suggest that you also change it's display property to one appropriate for relative positioning.