Confusion about invalid fixed settings for child elements in transform

The latest project is to achieve a click to show the effect of hiding the sidebar, And the sidebar needs to scroll with the scroll, the idea is simple, not a css animation and a position is fixed, but do not want, the sub-element set as fixed cannot be fixed, and this will subvert me. After painstaking efforts and careful troubleshooting, we finally found that the fixed failure caused by transform does not seem to have a solution unless it is bypassed. At the same time, I am completely aware of the gap between Baidu and google. Two Articles have mentioned that this address is Baidu!