[FIX] web: CalendarView: event edition in month mode

Fixes the way record end hours are computed in calendar views.

Before this commit:  objects `record start` and `record end` properties being copied by reference
and being incorrectly rewritten by other functions.
This caused incorrect end hour computations (and a SQL crash in the Planning app).

Now, objects are copied by value and don't interfere with each other.

Task 2072961

closes odoo/odoo#37690

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>

addons/web/static/src/js/views/calendar/calendar_model.js

@@ -707,8 +707,8 @@ return AbstractModel.extend({
             'record': evt,
             'start': date_start,
             'end': date_stop,
-            'r_start': date_start,
-            'r_end': date_stop,
+            'r_start': date_start.clone(),
+            'r_end': date_stop.clone(),
             'title': the_title,
             'allDay': all_day,
             'id': evt.id,

addons/web/static/tests/views/calendar_tests.js

@@ -2619,6 +2619,48 @@ QUnit.module('Views', {
         calendar.destroy();
     });
 
+    QUnit.test("drag and drop on month mode", async function (assert) {
+        assert.expect(2);
+
+        const calendar = await createCalendarView({
+            arch:
+                `<calendar date_start="start" date_stop="stop" mode="month" event_open_popup="true" quick_add="False">
+                    <field name="name"/>
+                    <field name="partner_id"/>
+                </calendar>`,
+            archs: archs,
+            data: this.data,
+            model: 'event',
+            View: CalendarView,
+            viewOptions: { initialDate: initialDate },
+        });
+
+        // Create event (on 20 december)
+        var $cell = calendar.$('.fc-day-grid .fc-row:eq(3) .fc-day:eq(2)');
+        testUtils.triggerMouseEvent($cell, "mousedown");
+        testUtils.triggerMouseEvent($cell, "mouseup");
+        await testUtils.nextTick();
+        var $input = $('.modal-body input:first');
+        await testUtils.fields.editInput($input, "An event");
+        await testUtils.dom.click($('.modal button.btn-primary'));
+        await testUtils.nextTick();
+
+        // Move event to another day (on 19 december)
+        await testUtils.dragAndDrop(
+            calendar.$('.fc-event:contains("An event")'),
+            calendar.$('.fc-day-grid .fc-row:eq(3) .fc-day-top:eq(1)')
+        );
+        await testUtils.nextTick();
+        await testUtils.dom.click(calendar.$('.fc-event:contains("An event")'));
+
+        assert.containsOnce(calendar, '.popover:contains("07:00")',
+            "start hour shouldn't have been changed");
+        assert.containsOnce(calendar, '.popover:contains("19:00")',
+            "end hour shouldn't have been changed");
+
+        calendar.destroy();
+    });
+
     QUnit.test('drag and drop on month mode with date_start and date_delay', async function (assert) {
         assert.expect(1);