Twenty fifteen theme – change responsive menu condition

Check out twentyfifteen/js/functions.js line 86

Try to play around with the values a bit – the code is pretty well documented.

Also don’t forget to clear your browser cache when updating css/js files (just to be sure).

EDIT

Have you looked at style.css line 4097
Specifically this part:

@media screen and (min-width: 59.6875em) {

The .sidebar class is defined there, anything under 59.6875em doesn’t style the sidebar at all.

You might want to consider changing the min-width value to something like 1080px (or whatever number you need).

Lmk if that does it.