Backport #25231 by @wxiaoguang
It causes not only one issue like #25221 (the footer width was also
affected by that change and was fixed some time ago)
The problem of "overflow: overlay" (#21850) is:
* It's not widely supported and is non-standard
https://caniuse.com/css-overflow-overlay
* It's not widely tested in Gitea (some standard layout like `ui
container + ui grid` may break it).
* The benefit seems smaller than the problems it brings.
So, I think it is good to revert it.
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Let's leave enough time for testing and reviewing.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
