feat: fix invalid droptarget at bottom of tree (#363)

Author: lukasbach

next-release-notes.md

@@ -1,4 +1,5 @@
 ### Bug Fixes and Improvements
 - If a tree environment renders without an item defined as focused in its `viewState` parameter, it will invoke the `onFocusItem`
   prop with the first item in the tree during its render. In the past, this was implicitly and silently set in the `viewState` prop,
-  now this assignment is triggered explicitly with the handler call (#363)
+  now this assignment is triggered explicitly with the handler call (#363)
+- Fixed a bug where an additional invalid drop target would be available at the bottom-most location when dragging via keyboard interactions (#363)

packages/core/src/drag/DragAndDropProvider.tsx

@@ -282,7 +282,7 @@ export const DragAndDropProvider: React.FC<React.PropsWithChildren> = ({
     if (environment.activeTreeId) {
       setProgrammaticDragIndex(oldIndex =>
         Math.min(
-          viableDragPositions[environment.activeTreeId!].length,
+          viableDragPositions[environment.activeTreeId!].length - 1,
           oldIndex + 1
         )
       );