Fix icon position of sidebar-search-container

[bastimeyer]

If a customized html_sidebars list is used and scroll-start is set before search, then scrolling down the sidebar will also scroll the searchbox's icon, as its position is set to absolute. By setting the position of the searchbox container to relative, the icon stays at its intended position while scrolling.

Example of the sidebar scrolled down by a bit and the icon having a vertical offset:

[bastimeyer]

For some reason, the position: relative property is not included in the built furo.css file of beta20 wheel file. It is present on the theme's docs, though, which is weird.

$ curl -s 'https://pradyunsg.me/furo/_static/styles/furo.css' | grep -Eo '\.sidebar-search-container\{[^}]+\}'
.sidebar-search-container{background:var(--color-sidebar-search-background)}
.sidebar-search-container{display:flex;align-items:center;margin-top:var(--sidebar-search-space-above);position:relative}

$ curl -o furo-b20.whl 'https://files.pythonhosted.org/packages/b0/9f/d00f11e59f5972c19b1de1d3fce06c51aaa1067299cdba5fcc2b759a05f8/furo-2020.12.9b20-py3-none-any.whl'
$ unzip -p furo-b20.whl furo/theme/static/styles/furo.css | grep -Eo '\.sidebar-search-container\{[^}]+\}'
.sidebar-search-container{background:var(--color-sidebar-search-background)}
.sidebar-search-container{display:flex;align-items:center;margin-top:var(--sidebar-search-space-above)}

[pradyunsg]

It is present on the theme's docs, though, which is weird.

It is. :/

I built it in a clean environment, so... that's confusing. I've gone ahead and cut beta21, with the correct files now. I'll debug further later. :)