fix: unchecked child repositions after its parent and siblings

When unchecking a child from the done section, it stayed at the end of
the items array (where done items live), appearing as the last active
item instead of under its parent. Now repositions after the last sibling
or parent in the array.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/utils/checklist.test.ts

@@ -112,6 +112,21 @@ describe('toggleItemWithCascade', () => {
 		expect(result[0].checked).toBe(false);
 		expect(result[0].parentId).toBeNull();
 	});
+
+	it('unchecking a child repositions it after its parent and siblings', () => {
+		// Simulates: parent + 2 children, one child checked (at end of array as done item)
+		const items: ChecklistItem[] = [
+			{ id: '1', text: 'Parent', checked: false, parentId: null },
+			{ id: '3', text: 'Child B', checked: false, parentId: '1' },
+			{ id: '4', text: 'Other', checked: false, parentId: null },
+			{ id: '2', text: 'Child A', checked: true, parentId: '1' }  // done, at end
+		];
+		const result = toggleItemWithCascade('2', items);
+		// Child A should be repositioned after Child B (last sibling), not stay at end
+		const ids = result.map(i => i.id);
+		expect(ids).toEqual(['1', '3', '2', '4']);
+		expect(result.find(i => i.id === '2')!.checked).toBe(false);
+	});
 });
 
 describe('indentItem', () => {

src/lib/utils/checklist.ts

@@ -31,11 +31,29 @@ export function toggleItemWithCascade(id: string, items: ChecklistItem[]): Check
 		return item;
 	});
 
-	// When unchecking a child from done: if parent no longer exists, orphan it
+	// When unchecking a child: reposition it after its parent/siblings in the array
+	// (otherwise it stays at the end of the array where done items live)
 	if (!newChecked && target.parentId) {
 		const parentExists = result.some((i) => i.id === target.parentId);
 		if (!parentExists) {
+			// Parent deleted — orphan it
 			result = result.map((i) => (i.id === id ? { ...i, parentId: null } : i));
+		} else {
+			// Remove the item from its current position
+			const uncheckedItem = result.find((i) => i.id === id)!;
+			result = result.filter((i) => i.id !== id);
+			// Find the last sibling or the parent, insert after it
+			let insertAfter = -1;
+			for (let i = 0; i < result.length; i++) {
+				if (result[i].id === target.parentId || result[i].parentId === target.parentId) {
+					insertAfter = i;
+				}
+			}
+			if (insertAfter >= 0) {
+				result.splice(insertAfter + 1, 0, uncheckedItem);
+			} else {
+				result.push(uncheckedItem);
+			}
 		}
 	}