(Apologies that this bug has been reported before, but this ticket actually describes the 1-line solution, so please don't close my ticket)

Bug

If a client finishes/aborts breaking a block, then starts breaking a block at the same position, the client will fail to send START_BREAKING_BLOCK. This results in client/server desync - the client is breaking a block but the server doesn't realize it.

Vanilla Reproducer

The easiest way to see this bug in action (using a vanilla client/server), is to make a command block that continually sets a block, then break it while in survival: https://youtu.be/u4nn48TTpkE

Code Analysis / Fix

The fix for this bug is very simple:

The check `this.sameDestroyTarget(var1)` inside the method `MultiPlayerGameMode#continueDestroyBlock` should be replaced with `this.sameDestroyTarget(var1) && this.isDestroying`

This simple change will make the client send START_BREAKING_BLOCK, informing the server that the client intends to start breaking a block.

Addendum

This bug occurs very frequently and is the root cause of a lot of "breaking animation" glitches, some of which date back to Jan 2013:

MC-8472 (adventure mode CanDestroy)
MC-19676 (survival -> adventure mode breaking)
MC-69865 (refocusing same block)
MC-157404 (breaking block behind insta-break block)

... and over 50 more