jquery - keep window from changing scroll position while prepending items to a list?

Store a reference to the first message before you prepend new messages, and after you prepend, set the scroll to the offset of that message:

$(document).on('scroll', function() {
    var scroll = $(document).scrollTop();
    if (scroll < 1) {
        // Store eference to first message
        var firstMsg = $('.message:first');

        // Prepend new message here (I'm just cloning...)
        $('body').prepend(firstMsg.clone());

        // After adding new message(s), set scroll to position of
        // what was the first message
        $(document).scrollTop(firstMsg.offset().top);
    }
});

Demo: http://jsfiddle.net/GRnQY/

Edit: I noticed you wanted it with a button. You might have to do a little more math:

$(document).on('click', '#loadMore', function() {
    var firstMsg = $('.message:first');

    // Where the page is currently:
    var curOffset = firstMsg.offset().top - $(document).scrollTop();

    // Prepend
    firstMsg.before(firstMsg.clone());

    // Offset to previous first message minus original offset/scroll
    $(document).scrollTop(firstMsg.offset().top-curOffset);
});

Demo: http://jsfiddle.net/GRnQY/5/

No need for jQuery here, just check for changes in the elements scrollHeight and adjust the scrollTop accordingly, ie:

var lastScrollHeight = el.scrollHeight;
for(var n=0; n<10; n++){
    // Prepend here...
}
var scrollDiff = el.scrollHeight - lastScrollHeight;
el.scrollTop += scrollDiff;

Demo

http://jsfiddle.net/fkqqv28e/

jQuery

To access the native DOM node from a jQuery collection, use array access; ie:

var lastScrollHeight = $('#myElement')[0].scrollHeight;
for(var n=0; n<10; n++){
    $('#myElement').prepend('<div>prepended '+Math.random()+'</div>');
}
var scrollDiff = $('#myElement')[0].scrollHeight - lastScrollHeight;
$('#myElement')[0].scrollTop += scrollDiff;

Some folks coming here may just want to add content without the current focus jumping around. Modifying Jeff B's demo above to use the click event target makes this idiom more portable:

someButton.on('click', function() {
    var curOffset = $(this).offset().top - $(document).scrollTop();
    // add content to your heart's content.
    $(document).scrollTop($(this).offset().top-curOffset);
});

c.f. http://jsfiddle.net/bwz8uLvt/1/ (variant of Jeff B's demo above but with the [add more] button at an arbitrary point in the page).