Look again index, index actually found some contraindications and precautions;
Only valid index of positioning elements;
In the case of two overlapping elements:
For siblings:
1:
Positioning ------- consistent
uniform z-index ---------
Order after the cover appeared first appears
2：
------- positioning the positioning element positioned elements overlap not
uniform z-index ---------
------- consistent order
3：
--------- consistent positioning
z-index --------- large value to cover a small value
consistent sequence -------
For father and son element
1: active parent element z-index:
sub-elements parent element covering
2: parent element z-index failure (z-index is not provided or is not positioned):
sub-subject element z-index;
Reproduced in: https: //www.jianshu.com/p/85338f03457f