Rollup merge of #109932 - GuillaumeGomez:source-code-scrollbar, r=notriddle

Source code scrollbar

Fixes https://github.com/rust-lang/rust/issues/109865.

You can test it [here](https://rustdoc.crud.net/imperio/source-code-scrollbar/src/test_docs/lib.rs.html).

It seems to introduce a few regressions (at least change of behaviour) about the scroll "saving" though.

r? `@notriddle`

src/librustdoc/html/static/css/rustdoc.css

@@ -392,6 +392,7 @@ img {
 	overflow-x: hidden;
 	/* The sidebar is by default hidden  */
 	overflow-y: hidden;
+	z-index: 1;
 }
 
 .sidebar, .mobile-topbar, .sidebar-menu-toggle,
@@ -535,6 +536,9 @@ ul.block, .block li {
 .rustdoc .example-wrap > pre {
 	margin: 0;
 	flex-grow: 1;
+}
+
+.rustdoc:not(.source) .example-wrap > pre {
 	overflow: auto hidden;
 }