How can I prevent body scrollbar and shifting when twitter bootstrap modal dialog is loaded?

Question

Marko,

I just had the same problem. It appears that Bootstrap 3.0.0 adds a class to <body>, modal-open, when the modal first shows. This class adds margin-right: 15px to the body to account for a scrollbar, which is there on longer pages. This is great, except for shorter pages when a scrollbar isn’t on the body. In the no scrollbar case, the margin-right causes the body to shift left on modal open.

I was able to solve this by adding some short Javascript and a little CSS:

CSS:

/* override bootstrap 3 class to remove scrollbar from modal backdrop
   when not necessary */
.modal {
  overflow-y: auto;
}
/* custom class to override .modal-open */
.modal-noscrollbar {
    margin-right: 0 !important;
}

JS:

(function($) {

$(document)
    .on( 'hidden.bs.modal', '.modal', function() {
        $(document.body).removeClass( 'modal-noscrollbar' );
    })
    .on( 'show.bs.modal', '.modal', function() {
        // Bootstrap adds margin-right: 15px to the body to account for a
        // scrollbar, but this causes a "shift" when the document isn't tall
        // enough to need a scrollbar; therefore, we disable the margin-right
        // when it isn't needed.
        if ( $(window).height() >= $(document).height() ) {
            $(document.body).addClass( 'modal-noscrollbar' );
        }
    });

})(window.jQuery);

These combined permit the margin-right scrollbar fix to work for long pages, yet is disabled for shorter pages (when document height <= window height). I hope this helps!

Bootstrap 3.0.1+ (tested up to 3.1.1) is a different story. Try the following:

CSS:

/* override bootstrap 3 class to remove scrollbar from modal backdrop
   when not necessary */
.modal {
  overflow-y: auto;
}
/* custom class to add space for scrollbar */
.modal-scrollbar {
    margin-right: 15px;
}

JS:

(function($) {

$(document)
    .on( 'hidden.bs.modal', '.modal', function() {
        $(document.body).removeClass( 'modal-scrollbar' );
    })
    .on( 'show.bs.modal', '.modal', function() {
        // Bootstrap's .modal-open class hides any scrollbar on the body,
        // so if there IS a scrollbar on the body at modal open time, then
        // add a right margin to take its place.
        if ( $(window).height() < $(document).height() ) {
            $(document.body).addClass( 'modal-scrollbar' );
        }
    });

})(window.jQuery);

EDIT:

In light of Mac eliminating scrollbars from inhabiting window render width, here’s a more portable solution (3.0.1+) if you don’t mind some feature detection. Reference: http://davidwalsh.name/detect-scrollbar-width

CSS:

.scrollbar-measure {
    height: 100px;
    overflow: scroll;
    position: absolute;
    top: -9999px;
}

JS:

window.jQuery(function() {
  // detect browser scroll bar width
  var scrollDiv = $('<div class="scrollbar-measure"></div>')
        .appendTo(document.body)[0],
      scrollBarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;

  $(document)
    .on('hidden.bs.modal', '.modal', function(evt) {
      // use margin-right 0 for IE8
      $(document.body).css('margin-right', '');
    })
    .on('show.bs.modal', '.modal', function() {
      // When modal is shown, scrollbar on body disappears.  In order not
      // to experience a "shifting" effect, replace the scrollbar width
      // with a right-margin on the body.
      if ($(window).height() < $(document).height()) {
        $(document.body).css('margin-right', scrollBarWidth + 'px');
      }
    });
});

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