How to only display the main window

You can set panels size from config file.

To make only the right panel visible, just change splitterWidth and splitterMinWidth to 0.

Rendering result will be that one. User will still be able to make visible left panels by dragging the Splitter grip.

See also : https://github.com/servocoder/RichFilemanager/blob/master/scripts/filemanager.config.default.json#L21