There have been a lot of major changes made to the way GIMP works, and the developers have been keen to promote the new single window mode especially. It’s been a feature requested by the community for a long time, being able to integrate the dockable toolbars into the main GIMP window. While the floating windows were created originally to emulate the look of Photoshop, they never really worked the same way, and there was plenty of times when we just lost the bars as they suddenly moved off screen during start up. The new, completely optional, mode smartly places the tool bars in the main window, allowing you to drag them around and modify completely as you see fit, with multiple columns and different tools. It works great once you get used to the little quirks.