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This is the Show-Hide/Collapse-Expand WordPress plugin from (click on the power plug/mains icon in the dashboard) from Version 1.3.0 | By Bunte Giraffe | View details. It works here, but not in large pages (where it’s needed). Could there be some incompatible code in those pages?
https://bunte-giraffe.de/show-hidecollapse-expand-faq-documentation/ No help with bugs.
https://wordpress.org/support/plugin/show-hidecollapse-expand/page/2/ has some issues. Not organized well.