grid-layout drag from out side shake's bug

[wxwebfeng]

Software version (please complete the following information):

• Browser [e.g. chrome, safari]
• Vue Version [e.g. 2.5.7]
• vue-grid-layout Version: [e.g. 2.4.0]

Describe the bug After you drag the component to the grid-layout region, an overlap with the grid-item component shows a jitter of what the bug is.

Please use the CodeSandbox Template to demonstrate your bug. It is much easier for us to help you if you do.

To Reproduce Steps to reproduce the behavior:

1. Go to 'https://jbaysolutions.github.io/vue-grid-layout/zh/guide/10-drag-from-outside.html'
2. Drag 'Droppable Element (Drag me!)' Element.
3. See that the 'v-show=isDragging' attribute of the element with a 'class' style of 'vue-grid-placeholder' is constantly switching between 'true' and 'false'

Expected behavior no

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Additional context