To reproduce:

Right-click version:

1. Clear your inventory
2. Place 16 signs in the middle hotbar slot
3. Place 8 signs in the first hotbar slot
4. Pick up the and right-click drag the stack of 8 signs across the entire hotbar
5. Expected behavior: The hotbar now contains 8 stacks of 1 sign and 1 stack of 16 signs. Actual behavior: There is one sign left with your cursor and one hotbar slot remains empty

Left-click version:

1. Clear your inventory
2. Place 16 signs in the first hotbar slot
3. Place 6 signs in your main inventory
4. Pick up the stack of 6 signs and and left-click drag them across the first three hotbar slots
5. Expected behavior: The hotbar now contains 1 stack of 16 signs and 2 stacks of 3 signs. Actual behavior: There are two signs left with your cursor and the newly placed stacks of signs each contain 2 instead of 3 signs