Constraint-based layout managers are more powerful than the common grid, grid-bag, and group layout managers. However, they are also more complex and come with potential problems such as over-constrained specifications and overlap in a GUI. Current GUI builders have little support for layout constraints, and it is not clear how such constraints can be made easily accessible to GUI designers. We will demonstrate a GUI builder - the Auckland Layout Editor (ALE) - that addresses these challenges, by allowing GUI designers to specify constraint-based layouts using only simple mouse operations. ALE guarantees that all operations lead to sound specifications, making sure that the layout is solvable and non-overlapping. To achieve the latter, we propose an algorithm that automatically generates the missing constraints that are necessary to keep a layout non-overlapping. Today's applications need to run on multiple devices with different screen sizes. For this a layout must have a good appearance at different sizes. To aid the designer in creating a layout with good resizing behavior, we propose a novel automatic layout preview, which displays the layout at its minimal and at an enlarged size chosen to visualize layout problems directly.