I know exactly what you mean. This is something Joomla! has to implement itself. The session cookie is set by the application long before any third party plugin is loaded. As a result, no third party plugin can change the cookies flags. Moreover, the cookie flags are set in Joomla's session management class. Even if a plugin could interact with it, there is no provision to add this flag.
I know exactly how to fix it, but I am currently blocked from contributing to Joomla. Therefore, not only can I not submit a patch, I cannot even report the issue. Please do go to issues.joomla.org and report the issue. Maybe if enough people report it they will finally fix it.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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