CSS not loading in fullcalendar when switching to tab

Question

I have a tab list panel and the last tab uses the full calendar

And there is this problem, when switching to the tab, the css of the calendar is not loading, but if I switch to another month of the calendar itself, the css starts working

My calendar version is 5.7.2, I did not observe such a problem for version 3, what could be the problem?

var data = [];

$(document).ready(function() {
    var calendarEl = document.getElementById('calendar');

    var calendar = new FullCalendar.Calendar(calendarEl, {
        initialView: 'dayGridMonth',
        headerToolbar: {
            left: 'prev',
            center: 'title',
            right: 'next'
        },
        editable: false,
        contentHeight: 705,
        events: data
    });

    calendar.render();
});

html, body {
  margin: 0;
  padding: 0;
  font-family: "Lucida Grande",Helvetica,Arial,Verdana,sans-serif;
  font-size: 14px;
}

#calendar {
  max-width: 900px;
  margin: 20px auto;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-eOJMYsd53ii+scO/bJGFsiCZc+5NDVN2yr8+0RDqr0Ql0h+rP48ckxlpbzKgwra6" crossorigin="anonymous">

<link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.10.0/css/all.css" integrity="sha384-AYmEC3Yw5cVb3ZcuHtOA93w35dYTsvhLPVnYs9eStHfGJvOvKxVfELGroGkvsg+p" crossorigin="anonymous" />

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/fullcalendar-scheduler@5.0.0-beta.1/main.min.css" />

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.11.1/moment.min.js" integrity="sha512-TAVtO5mj3yhqvqWq539ELZe1kdI+ggl2XkVNJ7h33EeYK03qTyiKPbTu2DrrsuWlggnvCr3+A29goA7y0aZSFg==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<script src="https://unpkg.com/fullcalendar-scheduler@5.7.2/main.js"></script>

<script src="https://code.jquery.com/ui/1.11.3/jquery-ui.min.js"></script>

<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta1/dist/js/bootstrap.bundle.min.js" integrity="sha384-ygbV9kiqUc6oa4msXn9868pTtWMgiQaeYH7/t7LECLbyPA2x65Kgf80OJFdroafW" crossorigin="anonymous"></script>

<nav>
  <div class="nav nav-tabs" id="nav-tab" role="tablist">
    <button class="nav-link active" id="nav-home-tab" data-bs-toggle="tab" data-bs-target="#nav-home" type="button" role="tab" aria-controls="nav-home" aria-selected="true">
                                Home
                            </button>
    <button class="nav-link" id="nav-profile-tab" data-bs-toggle="tab" data-bs-target="#nav-profile" type="button" role="tab" aria-controls="nav-profile" aria-selected="false">
                                Profile
                            </button>
    <button class="nav-link" id="nav-calendar-tab" data-bs-toggle="tab" data-bs-target="#nav-calendar" type="button" role="tab" aria-controls="nav-calendar" aria-selected="false">
                                Calendar
                            </button>
  </div>
</nav>
<div class="tab-content" id="nav-tabContent">
  <div class="tab-pane fade show active" id="nav-home" role="tabpanel" aria-labelledby="nav-home-tab">Home</div>
  <div class="tab-pane fade" id="nav-profile" role="tabpanel" aria-labelledby="nav-profile-tab">Profile</div>
  <div class="tab-pane fade" id="nav-calendar" role="tabpanel" aria-labelledby="nav-calendar-tab">
    <div id="calendar"></div>
  </div>
</div>

Answer 1

Call your function on 'shown.bs.tab' Your code runs after this tab is shown

var data = [];
var calendarEl = document.getElementById('nav-calendar-tab');
$(document).ready(function () {
  calendarEl.addEventListener('shown.bs.tab', function (event) {
   var calendarEl = document.getElementById('calendar');
      var calendar = new FullCalendar.Calendar(calendarEl, {
        initialView: 'dayGridMonth',
        headerToolbar: {
          left: 'prev',
          center: 'title',
          right: 'next'
        },
        editable: false,
        contentHeight: 705,
        events: data
      });
      calendar.render();
  })
});//end of document.ready

 var data = [];
var calendarEl = document.getElementById('nav-calendar-tab');
$(document).ready(function () {
  calendarEl.addEventListener('shown.bs.tab', function (event) {
   var calendarEl = document.getElementById('calendar');
      var calendar = new FullCalendar.Calendar(calendarEl, {
        initialView: 'dayGridMonth',
        headerToolbar: {
          left: 'prev',
          center: 'title',
          right: 'next'
        },
        editable: false,
        contentHeight: 705,
        events: data
      });
      calendar.render();
  })
});//end of document.ready

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-eOJMYsd53ii+scO/bJGFsiCZc+5NDVN2yr8+0RDqr0Ql0h+rP48ckxlpbzKgwra6" crossorigin="anonymous">

<link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.10.0/css/all.css" integrity="sha384-AYmEC3Yw5cVb3ZcuHtOA93w35dYTsvhLPVnYs9eStHfGJvOvKxVfELGroGkvsg+p" crossorigin="anonymous" />

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/fullcalendar-scheduler@5.0.0-beta.1/main.min.css" />

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.11.1/moment.min.js" integrity="sha512-TAVtO5mj3yhqvqWq539ELZe1kdI+ggl2XkVNJ7h33EeYK03qTyiKPbTu2DrrsuWlggnvCr3+A29goA7y0aZSFg==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<script src="https://unpkg.com/fullcalendar-scheduler@5.7.2/main.js"></script>

<script src="https://code.jquery.com/ui/1.11.3/jquery-ui.min.js"></script>

<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta1/dist/js/bootstrap.bundle.min.js" integrity="sha384-ygbV9kiqUc6oa4msXn9868pTtWMgiQaeYH7/t7LECLbyPA2x65Kgf80OJFdroafW" crossorigin="anonymous"></script>

<nav>
  <div class="nav nav-tabs" id="nav-tab" role="tablist">
    <button class="nav-link active" id="nav-home-tab" data-bs-toggle="tab" data-bs-target="#nav-home" type="button" role="tab" aria-controls="nav-home" aria-selected="true">
                                Home
                            </button>
    <button class="nav-link" id="nav-profile-tab" data-bs-toggle="tab" data-bs-target="#nav-profile" type="button" role="tab" aria-controls="nav-profile" aria-selected="false">
                                Profile
                            </button>
    <button class="nav-link" id="nav-calendar-tab" data-bs-toggle="tab" data-bs-target="#nav-calendar" type="button" role="tab" aria-controls="nav-calendar" aria-selected="false">
                                Calendar
                            </button>
  </div>
</nav>
<div class="tab-content" id="nav-tabContent">
  <div class="tab-pane fade show active" id="nav-home" role="tabpanel" aria-labelledby="nav-home-tab">Home</div>
  <div class="tab-pane fade" id="nav-profile" role="tabpanel" aria-labelledby="nav-profile-tab">Profile</div>
  <div class="tab-pane fade" id="nav-calendar" role="tabpanel" aria-labelledby="nav-calendar-tab">
    <div id="calendar"></div>
  </div>
</div>

Answer 2

It looks like your calendar is loading during page load, but the css is not being applied because the calendar element is hidden at that time. You can move the calendar load when a tab is clicked and specify a smaller interval, such as 500 milliseconds. Try the following code

<script>
   var data = [];

    $(document).ready(function () {
        $("#nav-calendar-tab").on("click", function () {
            setTimeout(function () {
                var calendarEl = document.getElementById('calendar');

                var calendar = new FullCalendar.Calendar(calendarEl, {
                    initialView: 'dayGridMonth',
                    headerToolbar: {
                        left: 'prev',
                        center: 'title',
                        right: 'next'
                    },
                    editable: false,
                    contentHeight: 705,
                    events: data
                });

                calendar.render();
            }, 500);
           
        });

    });//end of document.ready

   </script>