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

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
2. Create a large area of farmland, and place a stack of seeds in main hand
3. Try to plant the whole area, while right-clicking and walking around

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.