Improve sidebar behavior on inflexible websites

[fregante]

Sometimes, websites have fixed widths and don't adapt well when the sidebar is injected, which usually causes content to be unreachable. This was noted in https://github.com/pixiebrix/pixiebrix-extension/pull/4285#issuecomment-1247064084 as well.

I have a possible solution for this:

html {display: flex; overflow: hidden}
body {overflow-x: scroll}
.sidebar {position: sticky}

This makes the two views mostly independent from each other:

Elements with position: fixed could also be disabled via body {transform: rotate(0.1deg)}

These changes however would be more invasive than what we currently have, so they might cause trouble on some sites, even if they fix many issues on others.