Dragging a combo hides edges

### Describe the bug / 问题描述

when you enable the drag-element behavior and drag a combo, every link connected to its nodes is obscured by the layout of the combo, as if it had a lower z-index".


### Reproduction link / 重现链接

_No response_

### Steps to Reproduce the Bug or Issue / 重现步骤

1) it is possible to verify this behavior in the playground -> custom combo -> extra box by adding a background to the combo
for example: combos: [{ id: 'combo-0', style:{fill:'red', fillOpacity:1 }}],

2) then drag the combo

### G6 Version / G6 版本

🆕 5.x

### Operating System / 操作系统

Linux

### Browser / 浏览器

Chrome

### Additional context / 补充说明

_No response_

Start in the playground at custom combo → extra box and reproduce the issue by adding a red, opaque background to combo-0, then dragging the combo. Trace the drag-element behavior and combo rendering involved in that example. Done means links connected to the combo's nodes remain visible above or outside the combo background while it is dragged.

Written by the indexing model from the issue text.