How to synchronize the scroll offset of two elements during inertial scrolling

iOS actually freezes DOM manipulation and Javascript while inertial scrolling is happening. Here is a simple demo that I made to illustrate the difference between scrolling in a normal desktop environment vs the iPad: http://jsfiddle.net/notjoelshapiro/LUcR6/. This code only has this JS in it:

var num = 0;
function updateNum(){
    num++;
    $('#awesomeDiv').text(num);
}
$(window).scroll(updateNum);

On scroll it increments a number and displays it on the bottom of the page. You'll see that the scroll number at the bottom of the screen is only being incremented when the inertial scroll has stopped. If Javascript was acting in the background it wouldn't refresh until the scroll ended but the number should be incremented higher.

So to answer your questions specifically:

Is there any touch or scroll event fired during the inertial scrolling phase?

Negative, ghostrider.

Is there any way to run a Javascript callback during that phase?

See above.

Is there a way to sync the scroll offset of two elements (either X or Y, not both) using CSS or some other technology other than JS?

Not quite sure what you mean here, JS can do all of the math'ing that you need but it will have to be when inertial scroll has completed. To be honest though, this may be a symptom of tl;dr since I'm at work right now.

Have you looked at the iScroll library? It simulates intertial (or non-inertial) scrolling for touch and non-touch environments and gives you JS callbacks during/after "scroll" and "inertial scroll" and provides a lot of information that you can use to calculate where you are on the page.

OldDrunkenSailor beat me to suggesting iScroll.

Unfortunately, out of the box iScroll just replicates the same problem as native inertial scrolling -- there's no event handling during the inertial phase.

Here's a version of your demo with a monkey-patched iScroll to add a custom event that fires even during the inertial stage: https://dl.dropbox.com/u/15943645/scrollingdemo.html

Works great on my 2nd gen iPad.

JS:

// Disable touch events
document.addEventListener('touchmove', function (e) { e.preventDefault(); }, false);

// Patch iScroll for position change custom event
iScroll.prototype._oldPos = iScroll.prototype._pos;
iScroll.prototype._pos = function(x, y) {
    this._oldPos(x, y);
    if (this.options.onPositionChange) this.options.onPositionChange.call(this);
}

$(function() {
    var $win = $(window),
        $div_cols = $('#cols'),
        $div_rows = $('#rows'),
        $div_body = $('#body')

    // attach scrolling sync handler and execute it once
    function sync_scroll(e) {
        $div_cols.scrollLeft(0 - $div_body.position().left);
        $div_rows.scrollTop(0 - $div_body.position().top);

    }           

    // initialize iScroll on wrapper div, with position change handler
    var myScroll = new iScroll('iscroll_wrapper', {
        bounce: false,
        onPositionChange: sync_scroll
    });
})

CSS:

#iscroll_wrapper {
    position:absolute;
    z-index: 1;
    left: 168px; 
    top:77px; 
    bottom:0px; 
    right:0;
    overflow:auto;
}

#body {
    position:absolute;
    z-index: 1;
    width: 2046px; 
    height: 3376px;

}

Note only the body responds to touch events, but you can extend the technique to the rows and cols divs for the reverse relationship.