Do you know if it is possible to create a custom sidebar like the one for "show sidebar"?
I know you can create a popup view, but it hovers above the web page - I am looking for a native kind of view like the "sidebar".
Safari does not support any native sidebar at the moment.
Chrome 114+ supports sidePanel, Firefox and Opera support sidebarAction. It's possible that in the future Safari may support sidePanel since it's a newer API, but I don't expect that to happen anytime soon.