Question

Fairly new to jQuery and JavaScript in general. I mocked up an example of my problem at http://jsbin.com/alibi3/2/ – with an explanation below.

I have a div that, after a user scrolls past a certain point on the page, is assigned a class of “fixed” so it follows the user down the page. This works fine on its own.

Problem is, the content above that div can be toggled to show/hide – and when it is shown, the fixed class is still being applied at the point it would have been if it was hidden, so it appears to ‘jump’.

How do I tell my fixed-class-adding function that the div above has been shown/hidden, and so adjust the point at which the ‘fixed’ class is added?

Thanks.

HTML:

<div id="drawer">
    <a href="#">Click here to toggle drawer</a>
    <p id="drawercontents">Here is the stuff in the drawer; hidden by default.</p>
</div>
<div id="article">
    blah, blah...
</div>
<div id="nav">
    This should follow down the page once we scroll past the start of the article,
    and 'stick' back in place when we are back at the top.
</div>

CSS:

  #article {
    width: 400px;
    float: left;
    margin-right: 20px;
    padding: 20px;
    background: #eee;
  }
  #nav {
    width: 200px;
    float: left;
    background: #ff0;
    padding: 20px;
  }
  #drawer {
    width: 660px;
    padding: 20px;
    color:#fff;
    background: #000;
    margin-bottom: 10px;
  }
  .fixed { position: fixed; left: 460px; top: 0px; }

JavaScript:

$(document).ready(function() {
  $('#drawercontents').hide();

  $('#drawer a').click(function(e){
    e.preventDefault();
    $('#drawercontents').toggle('fast');
  });

  var top =$('#nav').offset().top - parseFloat($('#nav').css('marginTop').replace(/auto/, 0));
  $(window).scroll(function () {
    // what is the y position of the scroll?
    var y = $(window).scrollTop();    
    // whether that's below the start of article?
    if (y >= top) {
      // if so, add the fixed class
      $('#nav').addClass('fixed');
    } else {
      // otherwise, remove it
      $('#nav').removeClass('fixed');
    }
  });
});

Answer 1

Whenever, you do something that modifies the base position of that “fixed” div, you need to re-snapshot its base position.

For example, in your demo code, remeasure the top inside the toggle() call.
See the modified code, below, or see it in action at http://jsbin.com/alibi3/8 .

var GblTop;

function GetVertOffset (srchStr)
{
    GblTop = $(srchStr).offset().top - parseFloat($(srchStr).css('marginTop').replace(/auto/, 0));
}


$(document).ready(function ()
{
    $('#drawercontents').hide();

    $('#drawer a').click (function (e)
    {
        e.preventDefault();
        $('#drawercontents').toggle('fast', function() {GetVertOffset ('#nav'); } );
    });


    GetVertOffset ('#nav');     //-- Sets GblTop

    $(window).scroll (function ()
    {
        // what is the y position of the scroll?
        var y = $(window).scrollTop();
        // whether that's below the start of article?
        if (y >= GblTop)
        {
            // if so, add the fixed class
            $('#nav').addClass('fixed');
        }
        else
        {
            // otherwise, remove it
            $('#nav').removeClass('fixed');
        }
    });

});