This appears to be a grey area, but for what it's worth, the intent as it reads to me is that if you're actively swinging your weapon or blocking with your shield, then you're not keeping the scroll steady enough to read it, and thus can't cast it. The passage is part of being able to see the scroll. If the scroll was attached to the top of your thigh and you're running, I'd rule the same thing ~ it's not in a readable position. Your legs still work, the shield still works, and you're weapons still work, but if you're moving them all higgeldy piggeldy due to combat, attached scrolls are therefore unreadable.