Question

I am trying to have a content block always be shown to the user, even if he scrolls way down the page. He should also be able to scroll up and down the content block. Here is a fiddle with a stripped down version to show you what I mean:

http://jsfiddle.net/9ehfV/2/

One should notice when scrolling down, until reaching the bottom of the red block, it will fix the block on the window, and when scrolling back up, it places it back.

In Firefox one can scroll up and down and the fixing/unfixing described above is imperceptible – smooth as silk.

Once one tries scrolling in Chrome or IE, though, it seems like the scroll event lags and one can see the block “glitching” for a second. It’s not code lag – it seems to be something with the browsers.

Is there any way to fix this? I’m at my wit’s end.

I’d appreciate suggestions on how I can achieve the same effect in a different way…thanks

Answer 1

Since JavaScript runs in the same thread as the UI, a scroll event callback can block the UI-thread and thus cause lag. You need to throttle the scroll event listener because some browsers fire a lot of them. Especially if you’re on OS X with an analog scroll device. Since you do a lot of height calculations in your listener, it will trigger a reflow (very expensive) for every scroll event that is fired.

To throttle the listener, you have to prevent the listener from firing every time. Usually you wait until the browser doesn’t trigger an event for x milliseconds, or have a minimum time between calling your callback. Try adjusting the value to see the effect. Even 0 milliseconds can help, since it will delay the execution of the callback until the browser has time (usually 5-40 ms).

It’s also a good practice to toggle a class to switch between states (static and fixed position) instead of hard-coding it in JavaScript. Then you have a cleaner separation of concerns and avoid potential extra redraws by mistake (see “Browsers are smart” section). (example on jsfiddle)

Wait for a pause of x ms

// return a throttled function
function waitForPause(ms, callback) {
    var timer;

    return function() {
        var self = this, args = arguments;
        clearTimeout(timer);
        timer = setTimeout(function() {
            callback.apply(self, args);
        }, ms);
    };
}

this.start = function() {
    // wrap around your callback
    $window.scroll( waitForPause( 30, self.worker ) );
};

Wait at least x ms (jsfiddle)

function throttle(ms, callback) {
    var timer, lastCall=0;

    return function() {
        var now = new Date().getTime(),
            diff = now - lastCall;
        console.log(diff, now, lastCall);
        if (diff >= ms) {
            console.log("Call callback!");
            lastCall = now;
            callback.apply(this, arguments);
        }
    };
}

this.start = function() {
    // wrap around your callback
    $window.scroll(throttle(30, self.worker));
};

jQuery Waypoints
Since you’re already using jQuery, I would have a look at the jQuery Waypoints plugin which has a simple and elegant solution to your problem. Just define a callback when the user scrolls to a certain waypoint.

Example: (jsfiddle)

$(document).ready(function() {
    // throttling is built in, just define ms
    $.waypoints.settings.scrollThrottle = 30;

    $('#content').waypoint(function(event, direction) {
        $(this).toggleClass('sticky', direction === "down");
        event.stopPropagation();
    }, {
        offset: 'bottom-in-view' // checkpoint at bottom of #content
    });
});