Sidebar scroll glitchy.

In some case, the sidebar doesn't expand to full page height, when browser height increases.

To repro:
1 open up the Chrome browser, resize it so that it's height is about half screen height, and it's width is about full screen width;
2 navigate to a Grav Learn2 theme page in the browser, for example: https://learn.getgrav.org/. The requirement here is that the sidebar should be long "enough" (about screen height), otherwise the issue won't manifest;
3 after the page is fully loaded, drag the bottom border of the browser to increase the page height;
4 the sidebar's height should grow; instead, it doesn't.

Start by reproducing the issue in Chrome on a Grav Learn2 theme page such as https://learn.getgrav.org/, using a sidebar that is about screen height. Inspect the theme's sidebar height styling and verify that increasing the browser height causes the sidebar to grow to the full page height.

Written by the indexing model from the issue text.