User interface layout is defined using constraints on the position and size of visual elements. Constraints may be explicit and/or implicit. Interactive design code helps designers create constraints without conflict or redundancy. The design code determines whether a proposed layout constraint is incompatible with established constraints, receives a designer's selection of an incompatible constraint, and displays user interface elements in previews of alternate layouts having different constraints. A designer can interactively explore possible resolutions of constraint incompatibilities as soon as the constraints are detectable. The design code animates a transition between possible layouts, highlights the incompatible constraints, and hides the compatible constraints. A subset of constraints may be shown, such as only the recently added constraints and closely related constraints.