Frames are depreciated and support will disappear in some release of the modern browsers.
You can emulate the behavior with html and css.
But there is no support for that layout in any modern forum software we reviewed. So it would require building custom Stuff which is what got us light years behind other forums in functionality.
We had 1500 people using frames. One of which was me. I still love that layout.
We searched for a left rail style plugin for discourse but it doesn’t not exist.
So the only option is to pay for custom work to build it and then hope it doesn’t break when regular updates to discourse come out.
Which is hard because it holds back improvements for the vast majority of users. Note only 15% of users use the site on desktop.
So I’m game if we can find a supported plugin that will not break everytime discourse updates and supports the gazillion features the full version does.
Meanwhile you can just command click thread titles which pops them out if you really want or use the back button on thread detail which will always return you to the exact spot you left off.