https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Containment
This affected layout a little and required adjustments to the CSS to
keep spacing the same. In particular, the margins of adjacent items
usually overlap with each other. However, when an item has contain:
layout, any margins of child nodes push out the size of the item itself.
This was making spacing between items a little too big. To solve that, I
removed margins in some places: in particular for certain classes that
often occur at the end of a `details.rustdoc-toggle` block, I removed
their bottom margin. Generally, the margins provided by the next item
down are sufficient.
Also remove an unnecessary margin-top on .code-header.
This commit ports the scroll locking behavior from the source sidebar to the
regular sidebar, and also fixes some bad behavior where opening a "mobile"
sidebar, and growing the viewport so that the "desktop" mode without scroll
locking is activated, could potentially leave the page stuck.
This does not affect the behavior on larger screens. Only small ones, where
the sidebar covers up the main content.
