当松开鼠标按钮时，被拖动的元素和拖放到的目标元素就会互换位置，而且所有透明矩形也会消失，直到下一次切换。
When you release the mouse button, the dragged element and the drag target switch places, and all translucent rectangles disappear until the next swap.
当拖放时，只有这个透明的矩形会移动—初始的元素保持不动直到鼠标按钮被松开为止。
While you're dragging, only the translucent rectangle moves-the original element stays put until the mouse button is released.
试想一下这样的情况：用户在某个按钮上按下鼠标，发现错了，立即松开鼠标而不是释放鼠标。
Think about what happens when users press their mouse on a button, realize it's a mistake and move their mouse off of the button before releasing it.
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