Last night I built a farm that uses portals to spawn mobs. While testing it I decided to repair my items with XP from the farm. I started at around level 24, repaired an axe, shovel, and two pickaxes, then checked on my armor. It still had a little bit of damage, so I kept collecting XP. I got all the way up to level 45 and my armor still had damage. 

Confused, I took off all my armor and held a piece while collecting XP. I didn't get the little "drop" animation that seems to imply Mending is working. But when I put the armor back on, the damage indicator was gone. It was also gone on my other armor. So I can't tell if it's a visual glitch (the damage bar doesn't update) or if you have to hold armor for it to be affected by Mending, but neither is what I expected.