Introduce a new WaterfallPanel control for Avalonia and a demo UserControl showcasing its usage. The panel supports Groups, AutoGroup, DesiredLength (coerced to >= 0) and Orientation; AutoGroup computes columns/rows based on DesiredLength. Measure/Arrange logic distributes children into the shortest group (waterfall layout) and sizes items by dividing the primary axis by group count. Also add WaterfallPanelDemo.axaml and code-behind demonstrating horizontal/vertical examples with a NumericUpDown to adjust groups.
NaBian
22cbc47156