That window is particularly sensitive to free-floating when you try to click anywhere on the upper bar. Using the UI-Dots as suggested will come in handy to get things back to where you want them. Another way to get it back into place is to double-click the small, black downward arrow in the upper right-hand corner, which will immediately dock it into place at the correct size. Essentially, you will be reselecting Docked in the contextual menu by doing that.