fix(course-outline): correct library drag targets

Author: navinkarkera

src/course-outline/OutlineNode.tsx

@@ -339,9 +339,11 @@ const OutlineNode = ({
     ) :
     undefined;
 
-  const isDroppable = depth === 2
-    ? (parentSubsection as any)?.actions?.childAddable ?? false
-    : actions.childAddable || (parentSection?.actions?.childAddable ?? false);
+  // Node is droppable when it can be a same-level reorder target or can accept children
+  const isDroppable = Boolean(actions.draggable
+    || actions.childAddable
+    || (depth === 1 && parentSection?.actions?.childAddable)
+    || (depth === 2 && parentSubsection?.actions?.childAddable));
 
   const showPaste = depth === 1 && blk.enableCopyPasteUnits && showPasteUnit && sharedClipboardData;
 

src/course-outline/drag-helper/DraggableList.test.tsx

@@ -260,6 +260,43 @@ describe('DraggableList — drag-local tree ref', () => {
     });
   });
 
+  describe('unit drag over non-childAddable subsection via contained-unit collision', () => {
+    it('must not preview or commit when over unit inside childAddable=false subsection', () => {
+      // sub-B has childAddable=false (e.g. library subsection) but draggable=true
+      const unitA = makeUnit('unit-A');
+      const unitB = makeUnit('unit-B');
+      const subsectionA = makeSubsection('sub-A', [unitA]);
+      const subsectionB = makeSubsection('sub-B', [unitB]);
+      subsectionB.actions.childAddable = false;
+      const section1 = makeSection('sec-1', [subsectionA]);
+      const section2 = makeSection('sec-2', [subsectionB]);
+      const items = [section1, section2];
+
+      const callbacks = renderList(items);
+
+      expect(mockDndHandlers.current).not.toBeNull();
+
+      // Start dragging unit-A from sub-A.
+      fireDragStart('unit-A', 'Unit A', 'vertical');
+
+      // Drag over unit-B inside sub-B (childAddable=false).
+      fireDragOver(
+        'unit-A',
+        'unit-B',
+        { category: 'vertical' },
+        { category: 'vertical' },
+      );
+
+      // unitDragOver guard should fire — no preview.
+      expect(callbacks.onPreviewTreeChange).not.toHaveBeenCalled();
+
+      // Drag end — no commit.
+      fireDragEnd('unit-A', 'unit-B');
+
+      expect(callbacks.onUnitDrop).not.toHaveBeenCalled();
+    });
+  });
+
   describe('drag cancellations reset ref', () => {
     it('resets drag-local state on cancel', () => {
       const section1 = makeSection('sec-1', [makeSubsection('sub-A')]);

src/course-outline/drag-helper/DraggableList.tsx

@@ -203,6 +203,7 @@ const DraggableList = ({
       activeInfo.parent?.id === overInfo.parent?.id
       || activeInfo.parent?.id === overInfo.item.id
       || (activeInfo.parent?.category === overInfo.category && !overInfo.item.actions.childAddable)
+      || (activeInfo.category === overInfo.category && !overInfo.parent?.actions.childAddable)
     ) {
       return;
     }