sticky inner-div resize to fill window

I have a site with header and footer, and the main content is a large text column and a side column. An example of something similar: https://www.ourcommons.ca/DocumentViewer/en/43-2/house/sitting-69/hansard
The side column is short and sticky, so it goes with the window. I gave it a grey border to make a box around it, but the bottom of the box is right under the content, instead of at the bottom of the window like in the example. I think I need to set the height of the sticky div based on the window position or something, but I can't figure it out...