For me, speedbar is a no-go, since it opens in a new frame. If I switch to browser, and then back to Emacs buffer, it is still hidden behind a browser windows.

From sidebar, what I look is an exact equivalent of Sublime Text's sidebar.

- It opens in the same frame - I can add/pin a folder tree there, it should be operable by mouse (open/close subtree, click to open a file in buffer or switch to it) - Have a list of open files / buffers - clicking on it should switch to buffer

Currently looking at treemacs and neotree that were suggested by me. Previously I was giving up trying to configure them, now I may be more persistent.