Prevent select2 from flipping the dropdown upward

You can do it by overwriting CSS like so:

.select-dropdown {
  position: static;
}
.select-dropdown .select-dropdown--above {
      margin-top: 336px;
}

Since modifying the source code is not an option and adding a hook to the select2:open event is not very elegant, especially when you have multiple select2 instances in the same page, I have written a small extension for the Select2 plugin.

My implementation is inspired by a PR from the plugin's repository (https://github.com/select2/select2/pull/4618) that is not yet merged.

Basically, the following code overrides the original plugin function that handles the dropdown positioning and adds a new option (dropdownPosition) to force the dropdown positioning above/below.

The new dropdownPosition option can take the following values: - below - the dropdown is always displayed at the bottom of the input; - above - the dropdown is always displayed at the top of the input; - auto (default) - it uses the old behavior.

Just insert the following code after select2.js file:

(function($) {

  var Defaults = $.fn.select2.amd.require('select2/defaults');

  $.extend(Defaults.defaults, {
    dropdownPosition: 'auto'
  });

  var AttachBody = $.fn.select2.amd.require('select2/dropdown/attachBody');

  var _positionDropdown = AttachBody.prototype._positionDropdown;

  AttachBody.prototype._positionDropdown = function() {

    var $window = $(window);

    var isCurrentlyAbove = this.$dropdown.hasClass('select2-dropdown--above');
    var isCurrentlyBelow = this.$dropdown.hasClass('select2-dropdown--below');

    var newDirection = null;

    var offset = this.$container.offset();

    offset.bottom = offset.top + this.$container.outerHeight(false);

    var container = {
        height: this.$container.outerHeight(false)
    };

    container.top = offset.top;
    container.bottom = offset.top + container.height;

    var dropdown = {
      height: this.$dropdown.outerHeight(false)
    };

    var viewport = {
      top: $window.scrollTop(),
      bottom: $window.scrollTop() + $window.height()
    };

    var enoughRoomAbove = viewport.top < (offset.top - dropdown.height);
    var enoughRoomBelow = viewport.bottom > (offset.bottom + dropdown.height);

    var css = {
      left: offset.left,
      top: container.bottom
    };

    // Determine what the parent element is to use for calciulating the offset
    var $offsetParent = this.$dropdownParent;

    // For statically positoned elements, we need to get the element
    // that is determining the offset
    if ($offsetParent.css('position') === 'static') {
      $offsetParent = $offsetParent.offsetParent();
    }

    var parentOffset = $offsetParent.offset();

    css.top -= parentOffset.top
    css.left -= parentOffset.left;

    var dropdownPositionOption = this.options.get('dropdownPosition');

    if (dropdownPositionOption === 'above' || dropdownPositionOption === 'below') {
      newDirection = dropdownPositionOption;
    } else {

      if (!isCurrentlyAbove && !isCurrentlyBelow) {
        newDirection = 'below';
      }

      if (!enoughRoomBelow && enoughRoomAbove && !isCurrentlyAbove) {
        newDirection = 'above';
      } else if (!enoughRoomAbove && enoughRoomBelow && isCurrentlyAbove) {
        newDirection = 'below';
      }

    }

    if (newDirection == 'above' ||
    (isCurrentlyAbove && newDirection !== 'below')) {
        css.top = container.top - parentOffset.top - dropdown.height;
    }

    if (newDirection != null) {
      this.$dropdown
        .removeClass('select2-dropdown--below select2-dropdown--above')
        .addClass('select2-dropdown--' + newDirection);
      this.$container
        .removeClass('select2-container--below select2-container--above')
        .addClass('select2-container--' + newDirection);
    }

    this.$dropdownContainer.css(css);

  };

})(window.jQuery);

The initialize the plugin with as follows:

$(document).ready(function() {
  $(".select-el").select2({
    dropdownPosition: 'below'
  });
});

Fiddle here: https://jsfiddle.net/byxj73ov/

Github repository: https://github.com/andreivictor/select2-dropdownPosition

---

UPDATE December 30, 2019

Fiddle with the latest Select2 Version (v4.0.12): https://jsfiddle.net/g4maj9ox/

You can just edit select2.js


Where it says

enoughRoomBelow = dropTop + dropHeight <= viewportBottom,
enoughRoomAbove = (offset.top - dropHeight) >= this.body().scrollTop(),

just change it to

enoughRoomBelow = true,
enoughRoomAbove = false,