How to make sidebar sticky (like twenty fifteen theme)

I love the sticky sidebar feature in twenty fifteen, but there isn’t much else in that theme I like, and it isn’t very customizable.
I love what I’m reading about tinyframework (also, the number of issues in the forum marked ‘Resolved’ is really impressive) but I can’t find mention of this relatively rare feature. Can someone drop me a line about how to achieve that sticky sidebar functionality in tinyframework? I’ve tried the Q2W3 plugin already btw. Thank you.