I spent way too much time digging the correct properties. For future reference.

mousemove.ts

function onMouseMove(event, initialX: number, initialY: number) {
  const { clientX, clientY, movementX, movementY } = event
  
  const moving = {
    left: movementX < 0
    right: movementX > 0
    up: movementY < 0
    down: movementY > 0
  }
  
  // Use these to determine how much the cursor has moved from initialX & Y
  const xOffset = clientX - initialX // Negative value means left, positive means right
  const yOffset = clientY - initialY // Positive value means down, negative means up
  
  return {
    xOffset, yOffset,
    moving
  }
}