Fix for jump on top ( Bootstrap Modal - Fixed 100% height + width)

bootstrap_modal_fixed.css

html{
  overflow-x: hidden
}

body.modal-open{
  overflow: visible !important
  padding-right: 0 !important
}

.modal.full-size {
  position: fixed !important;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  transform: translate3d(0, 0, 0);
  -webkit-transform: translate3d(0, 0, 0);
}

.modal.full-size .modal-dialog {
  position: fixed;
  margin: 0;
  width: 100%;
  height: 100%;
  padding: 0;
}

.modal.full-size .modal-dialog .modal-content {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  border-radius: 0;
  box-shadow: none;
}

.modal.full-size .modal-dialog .modal-content .modal-body {
  position: absolute;
  height: 100%;
  width: 100%;
  font-weight: 300;
  overflow: auto;
}

.modal.full-size .modal-dialog .modal-content .modal-body .container-fluid {
  height: 100%;
}

bootstrap_modal_fixed.html

<div class="btn show-modal"> Book Now </div>
  <div id="bookOnline" class="modal fade full-size" role="dialog">
    <div class="modal-dialog">
      <div class="modal-content">
        <div class="modal-body">
          <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Error recusandae maxime repudiandae rerum molestias, officia provident inventore saepe eaque sint.</p>
        </div>
      </div>
    </div>
  </div>
</div>

bootstrap_modal_fixed.js

$(function() {
  $(".show-modal").on("click", function(e) {
    e.preventDefault();
    $('html').css('overflowY', 'hidden');
    $(".modal").modal('show');
  });

  $('.hide-modal').on("click", function(e) {
    e.preventDefault();
    $('html').css('overflowY', 'auto');
    $('.modal').modal('hide');
  });
});