<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="https://unpkg.com/fullcalendar@3.5.1/dist/fullcalendar.min.css" />
    <link rel="stylesheet" href="https://unpkg.com/fullcalendar-scheduler@1.7.1/dist/scheduler.min.css" />
    <script src="https://unpkg.com/jquery@2.1.4/dist/jquery.min.js"></script>
    <script src="https://unpkg.com/moment@2.19.1/min/moment.min.js"></script>
    <script src="https://unpkg.com/fullcalendar@3.5.1/dist/fullcalendar.min.js"></script>
    <script src="https://unpkg.com/fullcalendar-scheduler@1.7.1/dist/scheduler.min.js"></script>
    <style>
      .fc-view-container {
        overflow-x: scroll;
      }
    </style>
    <script>
      $(() => ((myCalendar) => {
        
        const handleHorizontalScrollbar = ((myCalendar) => {
        
          const minColumnWidthInPixels = 180; // up to you
      
          const getContainerWidth = () => myCalendar.parent().outerWidth();
  
          const getAgendaWidthInPercent = () => {
            const containerWidthInPixels = getContainerWidth();
            const numberOfColumns = myCalendar.fullCalendar("getResources").length;
            const firstColumnWidthInPixels = myCalendar.find(".fc-axis.fc-widget-header").outerWidth();
            const sumOfBorderWidthsInPixels = numberOfColumns;
            const expectedTotalWidthInPixels = minColumnWidthInPixels * numberOfColumns
                + firstColumnWidthInPixels
                + sumOfBorderWidthsInPixels;
            const agendaWidthInPercent = expectedTotalWidthInPixels / containerWidthInPixels * 100;
            return Math.max(agendaWidthInPercent, 100); // should not be less than 100% anyway
          }
          
          return (view) => {
            view.el.css("width", getAgendaWidthInPercent() + "%");
          };
          
        })(myCalendar);
      
        myCalendar.fullCalendar({
          defaultView: "agendaDay",
          viewRender: handleHorizontalScrollbar,
          windowResize: handleHorizontalScrollbar,
          resources: [
            { id: 0, title: "Room 0" },
            { id: 1, title: "Room 1" },
            { id: 2, title: "Room 2" },
            { id: 3, title: "Room 3" },
            { id: 4, title: "Room 4" },
            { id: 5, title: "Room 5" },
            { id: 6, title: "Room 6" },
            { id: 7, title: "Room 7" },
            { id: 8, title: "Room 8" },
            { id: 9, title: "Room 9" }
          ]
        });
        
      })($("#my-calendar")));
    </script>
  </head>

  <body>
    <div id="my-calendar"></div>
  </body>

</html>