Add scenario 10

spec/features/work_packages/datepicker/datepicker_logic_spec.rb

@@ -293,4 +293,26 @@ describe 'Datepicker modal logic test cases (WP #43539)',
       datepicker.expect_duration 4
     end
   end
+
+  describe 'when only due date set, set start date (test case 10)' do
+    let(:current_attributes) do
+      {
+        start_date: nil,
+        due_date: Date.parse('2021-02-11'),
+        duration: nil
+      }
+    end
+
+    it 'derives the duration' do
+      datepicker.expect_start_date ''
+      datepicker.expect_due_date '2021-02-11'
+      datepicker.expect_duration ''
+
+      datepicker.set_start_date '2021-02-09'
+
+      datepicker.expect_start_date '2021-02-09'
+      datepicker.expect_due_date '2021-02-11'
+      datepicker.expect_duration 3
+    end
+  end
 end