As part of the Desktop Improvements project we are making the sidebar menu collapsible. This presents the opportunity to think about other possible improvements or modifications to the sidebar's behavior. One idea that seems worth considering is making the sidebar sticky/fixed, such that it remains in-view as you scroll down the page. This behavior can currently be enabled via the FloatSide script (link).
Rationale for sticky/fixed
The sidebar menu is currently only available towards the top of the page. Giving people access to the menu regardless of where they are on the page would increase the utility of the menu.
In cases where the content of the sidebar menu are taller than the height of the screen the sticky/fixed sidebar would need to be able to scroll independently from the page content. This results in an interface that is more complex than the current one (two scrolling panes vs. one) which is not worth the possible additional utility. Additionally, in cases where people need to scroll the sidebar in order to reach menu contents below the fold those contents may be more difficult to access than they currently are.
Wait until we 1) release the collapsible sidebar, 2) move the language links out of the sidebar and then revisit this.
Scrolling: http://patchdemo.wmflabs.org/wikis/197e628bad08d9ea6ab286c6db366fe7/w/index.php/Main_Page (Code: Gerrit - 2 patches)
Scrolling with old layout: https://patchdemo.wmflabs.org/wikis/a880d24935e8a058b27dde56e6066f6b/w/ (Leaner code: Gerrit)