Precision: only affect off hand (survival/creative).

There are some problems with the blocks placement of the second hand.

First, it sometimes happens that some blocks can't be place (by no one) for no reason when a player is nearby (see beginning of the gif). We can see the block rollback on client side. At the same location it works with the main hand.

Then, when placing blocks quickly, we can see that the block disappears and reappears immediately, sometimes with a real rollback if many blocks were placed at the same time. It's annoying when a player bridges quickly and falls.

This problem has been occurring for several months now, and I believe it's also present from version 1.10 of Spigot.

If we use a protocolhack like ViaVersion or ProtocolSupport, the problem is always present and concerns all clients that can connect (1.9 / 1.10 / 1.11 / 1.12 ...) to server.

GIF: Gyazo gif (bug animation)