Fix deriving due date if altering start date in range (Fix scenario 14)

frontend/src/app/shared/components/datepicker/multi-date-modal/multi-date.modal.ts

@@ -585,7 +585,7 @@ export class MultiDateModalComponent extends OpModalComponent implements AfterVi
     const parsedEndDate = parseDate(this.dates.end || '') as Date;
 
     if (selectedDate < parsedStartDate) {
-      this.applyNewDates([selectedDate, parsedEndDate]);
+      this.applyNewDates([selectedDate]);
       this.setCurrentActivatedField('end');
     } else if (selectedDate > parsedEndDate) {
       if (activeField === 'end') {
@@ -616,6 +616,15 @@ export class MultiDateModalComponent extends OpModalComponent implements AfterVi
     if (this.dates.start && this.dates.end) {
       this.formUpdates$.next({ startDate: this.dates.start, dueDate: this.dates.end });
     }
+
+    // If only one is set, derive from duration
+    if (this.dates.start && !this.dates.end && !!this.duration) {
+      this.formUpdates$.next({ startDate: this.dates.start, duration: this.durationAsIso8601 });
+    }
+
+    if (this.dates.end && !this.dates.start && !!this.duration) {
+      this.formUpdates$.next({ dueDate: this.dates.end, duration: this.durationAsIso8601 });
+    }
   }
 
   private onDataChange() {

spec/features/work_packages/datepicker/datepicker_logic_spec.rb

@@ -574,20 +574,20 @@ describe 'Datepicker modal logic test cases (WP #43539)',
       datepicker.select_day 5
 
       datepicker.expect_start_date '2021-02-05'
-      datepicker.expect_due_date '2021-02-11'
-      datepicker.expect_duration 5
+      datepicker.expect_due_date '2021-02-10'
+      datepicker.expect_duration 4
 
       # Focus is on finish date
       datepicker.expect_due_highlighted
-      datepicker.select_day 10
+      datepicker.select_day 15
 
       datepicker.expect_start_date '2021-02-05'
-      datepicker.expect_due_date '2021-02-10'
-      datepicker.expect_duration 4
+      datepicker.expect_due_date '2021-02-15'
+      datepicker.expect_duration 7
 
-      apply_and_expect_saved duration: 4,
+      apply_and_expect_saved duration: 7,
                              start_date: Date.parse('2021-02-05'),
-                             due_date: Date.parse('2021-02-10')
+                             due_date: Date.parse('2021-02-15')
     end
   end