Items in your offhand don't visibly update their durability or do the item swap movement animation when using the mending enchantment while you have a chest or other inventory opened that doesn't show the offhand slot.

The items still receive experience and durability properly, but it isn't reflected in the hotbar slot until you close out of the chest or use the hotkey to move the item out of your offhand.

Other than the durability bar, this is also noticeable when using a resource pack to give tools different textures when at different durability levels. The texture isn't updated while the offhand slot is hidden while viewing another inventory.