Relatively Sticky

relatively sticky is a drop-in replacement for the excellent sticky-kit. The idea was to be able to have sticky elements within any scrollable element, not only window. But from the simple idea this grew into a rewrite of more or less the whole project.

License: WTFPL

Usage

$("#sticky_item").stick_in_parent()

You can optionally pass a hash of options to configure how Relativly Sitcky works. The following options are accepted, each one is optional:

parent : The element will be the parent of the sticky item. The dimensions of the parent control when the sticky element bottoms out. Defaults to the closest parent of the sticky element. Can be a selector.

scrollable : The element in which the sticky item should stick. Defaults to window. Can be a selector.

sticky_class : The name of the CSS class to apply to elements when they have become stuck. Defaults to "is_stuck".

offset_top : offsets the initial sticking position by of number of pixels, can be either negative or positive

bottoming : Boolean to control whether elements bottom out. Defaults to true

quick reminder of the "traditional" positioning algorithms

static : This is the "normal" positioning algorithm. top, right, bottom and left will have no effect.

relative : Works mostly like static, but the element will be displayed with the specified offset relative to its original position. All other elements will still behave as if there was no offset.

absolute : The element will be displayed with the specified offset relative to the next parent with a non-static positioning. All other elements will behave as if this element did not exist.

fixed : The element will be displayed with the specified offset relative to the window. All other elements will behave as if this element did not exist.

some notes on sticky positioning

Sticky positioning today is a commonly used feature in web development. For example, there is the affix class in twitter bootstrap, a jquery plugin called sticky-kit and there is even an effort to include it in the CSS standard.

The idea is that an element is displayed in normal flow as long as it is on the screen. But when you scroll, instead of leaving the screen, it "sticks" to it. This is especially usefull for navigation elements which need to be available all the time.

So sticky works like static by default and like fixed when it sticks to the screen. But that is fixed with a bit of relative because all other elements must still behave as if the sticky element would still be at its original position.

Now there are two approaches how sticky can be implemented if it is not available natively. The first approach is to use fixed. In that case you need to create a "spacer" element that will sit at the original position of the sticky element in order to make all other elements behave correctly.

The second approach is to use relative positioning. In that scenario, the position of the sticky element needs to be recalculated on every scroll event.

Most libraries (all I could find) use the first approach, probably because it has better performance. But there might be cases where the second approach is more suitable, for example scrollable child elements or fluid design. So this library implements exactly that.

Conclusion

Use native sticky positioning where possible (AFAIK Firefox and Safari).
Evaluate which approach works best for you in your specific usecase. Most likely, sticky-kit is the better choice. But maybe relatively-sticky actually works better for you.

Example:

var stick = function(element, modernizr, top) {
    if (modernizr.csspositionsticky) {
        element.css({
            position: 'sticky',
            top: top + 'px'
        });
    } else {
        element.stick_in_parent({
            offset_top: top
        });
    }
};

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