Page scrolls to top if left sidenav opens

When the left sidenav is opened, the page scrolls to the top instead of remaining stable.

Steps to reproduce:

1. View a long page in the channel manager
2. Scroll down to halfway the page
3. Open the left sidenav by clicking the toggle button in the top-left corner

Expected: the top of the scrolled-down page remains stable, i.e. the part that is visible right below the toolbar in the unscaled page is the same part that is visible right below the toolbar in the scaled-down page.
Actual: the page scrolls back to the top

There actually is quite some code to keep the scroll position stable, and this used to work fine while developing it (a substantial amount of hours was spent getting this right). It's unclear why this broke.