The bug
If you 'attack' air and then move to a block to mine, the pick wont instantly start to mine it, only after the attack indicator has almost run out.
How to reproduce
Hit air besides a block and then move to a block. This appears to happen for all tools, and it only seems to happen when you hold your mouse button after attacking, not when you click to attack and then try to mine.
Linked issues
Comments
Can confirm for 20w07a
Thank you for your report!
This ticket is being forward-resolved as a duplicate of MC-125129, as the latter contains a bit more info and videos demonstrating the issue.
Thanks again!
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Can confirm for 1.12.2
Note: It only seems to happen when you hold your mouse button after attacking, not when you click to attack and then try to mine.