It would be good if as well as providing a close button on side pane components a minimise or roll-up button were present, which causes the side pane to collapse to just its title bar. I often have several side panes open (search, groups, plugin sidepanes) and it's inefficient to continually scroll up and down, or find a small button somewhere else on the GUI to reopen the side pane.
Since plugins are disabled now and the search bar has been moved to the top of the entries table, many side panes components shouldn't be an issue any more. Thus I close this feature request. If you think that there is still room for improvement, please open a new issue at https://sourceforge.net/p/jabref/feature-requests/681/.