Question

I am really confused with this. Here I’ve got a div whose position should be fixed after certain scroll. Before position of that div is absolute. I tried it and found that for one of my screens fixing position is really making document less while on other screen it is not.

What I want to know is which of them is a bug? Does fixing position of element decreases document height?

Thanks

Answer 1

If you use a fixed positioning for an element, it is removed from the “normal layout flow”. The height of your document is determined by positioning of all the elements in the page and thus, all changes to positioning affect document height.

Fixed and absolute positioning are explained in detail in the CSS Visual formatting model spec: http://www.w3.org/TR/CSS2/visuren.html#absolute-positioning

Regarding absolute positioning (from the spec):

In the absolute positioning model, a box is explicitly offset with
respect to its containing block. It is removed from the normal flow
entirely (it has no impact on later siblings).

Pretty much the same applies for fixed positioning (Ibid.):

Fixed positioning is a subcategory of absolute positioning. The only
difference is that for a fixed positioned box, the containing block is
established by the viewport.