On fixed elements and backgrounds

Programming - Mar 31, 2024

After just playing with apsect-ratio and being pleasantly surprised at how intuitive it is, here’s an example of CSS acting unintuitively:

If you have a fixed element on your page, which means it doesn’t move when you scroll, you might realise that it no longer acts fixed if you apply a CSS filter on its nearest ancestor. Go ahead, try it on the CodePen.

This is because applying a filter on the fixed element’s immediate parent makes it becoming the containing block instead of the viewport.

Hui Jing has more to teach in there about scrolling, rendering performance, and trickery with using pseudo elements to avoid issues.

I find this kind of thing among the most challenging CSS concepts to wrap my mind around, like Block Formatting Contexts (BFCs). A BFC Is A Mini Layout In Your Layout. 🤯

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