Measure the required height of the left sidebar's main menu contents and size up the default window height accordingly
http://fortintam.com/blog/2011/06/24/dont-make-me-scroll/
When I have a ton of available vertical screen space, and considering that most control center panels' contents can benefit from it, then the control center window's default height should be set to the height of the headerbar + the measured height of the main (i.e. not expanded) menu's entries, so that there is no need to ever scroll the sidebar.
Avoiding a scrolling sidebar is particularly relevant for GNOME Control Center because that app might be used by less tech-savvy users and they might not expect the scrolling in that particular widget area. Adwaita's "ghost" (hover-only) scrollbars are not so easily discoverable by non-geeks and I don't want to be stuck with support calls all the time where the person can't find the "Peripherals >" and "Details >" entries because they're hidden below the invisible "scroll line"...