Did you disable cookies or something? Table of Contents sidebar hides and stays hidden across page loads, browser restarts, etc. Appearance sidebar is the same.

Did you disable cookies or something? Table of Contents sidebar hides and stays hidden across page loads, browser restarts, etc. Appearance sidebar is the same. Appearance sidebar lets you set the article width as well, with the default being slightly slim, but the wide option being the full browser width. The only way I can get it to reset is to clear my cookies, so its likely that.