As I have a Logitech G9 mouse that has 2 buttons on the left hand side I have programmed 'Button 4' to be Sprint and 'Button 5' to Swap Item.
But if I am holding Shift when I press 'Button 4' (Sprint) it acts as though it is the Swap Item (Button 5) that has been pressed.
Also when 'Button 4' (Sprint) is being held down and I right click to place something it move the item to the offhand, but it doesn't move it back by doing the same thing.
Resolving to MC-40227, now that the underlying bug has been determined.