This falls under the same issue as MC-87019, but while it was resolved for commands like /clear and /replaceitem, the mending enchantment still doesn't repair gear that is in a non-visible slot.
For clarification, I'm using non-visible slot to refer to a slot in the player's inventory that isn't currently visible due to having another inventory open. For example, offhand and armor slots aren't visible while looking in a chest (this is what made me notice the bug).
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In this case a non-visible slot is a slot in your inventory that you can't see because you have another inventory open. For example, your offhand and armor slots aren't visible while you're looking at the contents of a chest.
I can confirm this issue. I also wrote a "Steps to Reproduce":
Steps to Reproduce:
Step 1: Enchant a tool/weapon with Mending.
Step 2: Lose some durability of that tool/weapon.
Step 3: Put that tool/weapon in offhand.
Step 4: Place down any block that have GUI, such as chest.
Step 5: Throw some bottle o' enchanting.
Step 6: "Open" that block before the player can collect any xp.
Step 7: Wait until the player collect all xp. Now watch carefully about the tool/weapon in offhand.
Observed Behavior:
The tool/weapon still have a green durability bar, until the player "close" the block.
Expected Behavior:
The tool/weapon doesn't have a green durability bar.
Thank you for your report!
We're tracking this issue in MC-152526, so this ticket is being resolved and linked as a duplicate.
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I can't confirm this issue. I don't know what are "non-visible slots" mean?