ukoke

Sorry, the 2. sentence has to be: It “recloses“ only if I refresh the page.

WP Codex: creating a child theme
(Although I am not a code-writer, it seems to me that You have not completely underestood what means making a child theme.) It is said that we should NOT alter the CSS of the parent theme (when making a child theme so, then all the changes a reversible). If something breaks, we can simply change the THEME back into parent (and no need to reinstall WP! – relly it can not help, because we altered the css of a theme, not the version of WP). And we can add the altered css into css of child theme row by row, and save the changes, and refresh site in view tab after each add – then we will see at once if and where we have made some mistake:-)
With best wishes,
Juhan, Estonia, Europa

The theme supports only this one toggle sidebar. But we can easily add a second column inside the main area (i.e. to split the column into two columns). For exmple the plugin “Column-Matic” works well with this theme.
Also we can vary the content of the toggle sidebar per page (for example the plugins “IntelliWidget Per Page Featured Posts and Menus” and “Section Widget” work well. And also we can add a sticky menu on left/right side (for example with plugin(s) “Code flavours floating menu”, “Floating Menu”.)

Not very convenient, but if to decrease the zoom, some of the hidden objects on sidebar will be seen. (Contrariwise: we maybe need to take into consideration that visitor can view the site with zoom in (the default zoom of a browser for example 130 %), and it would be good that all the content of sidebar can be seen with this zoom also.
Not very consolatory, but it seems to me that the sidebar not scrolling is not a bug, but indispensable price for the toggle sidebar, and the advantage of this – when we read the lower part of a long page, we can always open and see the content of sidebar. Let’s try it with some other theme – we scroll down the text, and we see now only an empty lower area of sidebar.

In conclusion, a sidebar can be scrolling or not scrolling, but not both at same time:-)
(Of course, some plugins do not like such a nice sidebar also.)