Using GIMP for Windows you may have noticed that the toolbox is always on top. You maximise your image window and the toolbox stays on top obscuring part of your image. Some may argue that this give you access to the toolbar whenever you need it. Personally I find it annoying.
You can change this default behavior from Edit > Preferences > Window Management. Change the Hint for the toolbox from the default of Utility Window to Normal Window. You will be prompted to restart GIMP before the chages become effective