Layout managers are used to control the placement of widgets in graphical user interfaces (GUIs). Constraint-based layout managers are among the most powerful. However, they are also more complex and their layouts are prone to problems such as over-constrained specifications and widget overlap. This poses challenges for GUI builder tools, which ideally should address these issues automatically. We present a new GUI builder - the Auckland Layout Editor (ALE) - that addresses these challenges by enabling GUI designers to specify constraint-based layouts using simple, mouse-based operations. We give a detailed description of ALE's edit operations, which do not require direct constraint editing. ALE guarantees that all edit operations lead to sound specifications, ensuring solvable and non-overlapping layouts. To achieve that, we present a new algorithm that automatically generates the constraints necessary to keep a layout non-overlapping. Furthermore, we discuss how our innovations can be combined with manual constraint editing in a sound way. Finally, to aid designers in creating layouts with good resize behavior, we propose a novel automatic layout preview. This displays the layout at its minimum and in an enlarged size, which allows visualizing potential resize issues directly. All these features permit GUI developers to focus more on the overall UI design.