Overflowing Quirks

Much has been written about the inconsistencies of CSS within the various browsers. A great resource for learning about them is the QuirksMode website. Despite having read a lot of that site, I spent far too much time today trying to debug one.

Its worth mentioning upfront that this is with Firefox on my laptop. Since writing up this post, I have found that Firefox 1.0.x on my desktop does not behave as described here. I’m not sure which is infact ‘correct’.

For a website I’m developing in work, I have a drag-and-drop interface that allows the user to arrange objects represented as <div> elements onscreen. If the user cancels the drag, the <div> is animated returning to its original position. For reasons that are not important, the <div> can return to one of two containing <div> elements. This all works well, apart from the fact that for one of the containers, an object that is returning to it consistently ends up 1 pixel out of position.

I initially assumed this was to do with the Box model issues, but the css for the two containers have identical border declarations. A bit of further digging and I found the only difference was the overflow style - and funnily enough, thats exactly where the problem lies.

To demonstrate, below are two identical <div> elements that contain a child <div> which has position: relative and is offset by 5px on both top: and left:. The sole difference between them is the first has overflow: hidden, and the second overflow: visible. Each of the inner <div>s are armed with the following piece of javascript:

   var a=this.parentNode;
   this.innerHTML = 'dx:'+(this.offsetLeft-a.offsetLeft)+
                    ' dy:'+(this.offsetTop-a.offsetTop);

When you click on them, the offset between the child and its parent is shown. Remember that the css is identical for these examples in all but overflow.

click me
click me

As you can see, assuming you have clicked away and your are running the right level of the right browser, the offsets are different and its all thanks to the overflow. I have not fully investigated the various fixes for CSS issues such as Strict mode, but I was fortunately able to code a workaround using javascript that dynamically alters the overflow style depending on whether the user is dragging. I admit its not the cleanest solution.