The bug

Holding right-click prioritizes continuing to use off-hand item over main-hand item.

How to reproduce

1. Put a shield in off-hand and a hoe in the main hand
2. Create a large area of dirt
3. Try to till the dirt; once the block is no longer tillable (e.g. already turned into farmland) the shield will activate and tilling will stop until right click is released.

What happens: The first block of farmland you target gets planted, then you start blocking, and you don't stop blocking until you release the mouse button.

What I expected: blocking would stop once I was targeting a farmland block with nothing planted in it, and I would plant a seed in it.