Z-index is invalid when canvas width is greater than parent width

z-index fails when canvas width is greater than parent width
when doing a mobile web page, it is found that
canvas has the same settings width/height and parent

in css style.

z-index failure canvas is always on top
canvas width/height is set to 300 when canvas width/height is set to 600 in line. What is the problem with z-index?

Jun.17,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e97c08-47461.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e97c08-47461.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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