rustdoc: Source code sidebar should pin to selected file

[tesuji]

For example this doc: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_middle/macros.rs.html#2-7

When you click to the "<" button, it show the file structure and not scroll down to the current viewed file:

I expect to see rustdoc sidebar point to viewed file directly: