how to display submenu items when parent page active?

Thanks to everyone who created the Twenty Fifteen theme. To improve my child theme on GitHub, I would like submenu items to display whenever the parent/sibling page is active.

On startup, the menu should remain compact (as it currently does in TwentyFifteen), like this:

Page 1
Page 2
Page 3

When the user opens any top-level page, the menu should auto-expand to reveal submenu items, like this:

Page 1
– subpage a
– subpage b
Page 2
Page 3

In the standard Twenty Fifteen, these submenu items appear only when the user clicks specifically on the menu down-arrow (a small icon that most users do not see). Instead, my goal is for submenu items to appear whenever the parent page (or sibling page) is active, to show more context around the page.

I have read many posts that address related topics, but none address my particular issue. For example:
https://mpswp.wordpress.com/2015/06/27/twenty-fifteen-theme-reveals-hidden-submenus

Any suggestions on how to code this into my child theme?