How to dock toolbar

Super easy question (I think). How do I dock a toolbar? I had my ‘Content Browser’ docked next to ‘Modes’ on the left of my screen. I could drag it out and dock it in other positions as I required. I pulled it out of position last week and now it won’t dock in any other position, so it’s just floating as a separate window. Any idea how I can get it to snap back into a docked position.