@dmaza — I think there are two bugs when using IPv6 addresses. They may be two symptoms of the same problem. I have not seen them happen independently.

I haven't been able to recreate these issues with IPv4 addresses or usernames, but my testing was limited.) These were tested on latest Chrome/Mac

Bug 1 - Edit card background color does not display — This IP does not display the blue box around the IP's edit: https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=Trevor%20Bolliger&user=2602%3A306%3A302b%3A36c0%3A25f2%3A5923%3A4851%3A46a6&startDate=1501614000

Bug 2 - IPv6 edits always show in right column — IPv6 edits always show on the right (as in left/right, not as in 'correct') column:

https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=2602%3A306%3A302b%3A36c0%3A15ee%3A36eb%3A32b0%3A9ff8&user=Trevor%20Bolliger&startDate=1506884400 and https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=2602%3A306%3A302b%3A36c0%3A25f2%3A5923%3A4851%3A46a6&user=Trevor%20Bolliger&startDate=1501614000 and https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=2a02%3Ac7f%3Ab239%3A2000%3A318e%3A8e01%3A7765%3Adb97&user=2a02%3Ac7d%3A3c9c%3A1f00%3A492%3Ab562%3A18da%3A1544&startDate=1493665200&endDate=1498849200 and https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=2a02%3Ac7f%3Ab239%3A2000%3A318e%3A8e01%3A7765%3Adb97&user=LewisE&startDate=1493665200&endDate=1498849200

If you swap the first/second order of the IP and the other user you will see.

Note These problems are not happening here: https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=2601%3A243%3A404%3A3848%3A885D%3A5127%3AA4DA%3AA242&user=Miguelemejia&startDate=1456862400&endDate=1510171200

This is very strange. Looking into it