Re: [fxtf-drafts] [filter-effects-2] User feedback: backdrop-filter is way less useful because it requires children to not escape the "backdrop rectangle" (#408)

Without a live example and reference to the specification, it took me quite some time to find out what the issue actually is. As I understand it, @ell1e refers to this sentence in the ["Filtering and Clipping" section](https://drafts.fxtf.org/filter-effects-2/#backdrop-filter-operation):

> A computed value of other than none results in the creation of both a stacking context [CSS21] and a Containing Block for absolute and fixed position descendants, unless the element it applies to is a document root element in the current browsing context.

So in https://jsfiddle.net/SebastianZ/jp5ts2mv/ the "Headline" text is positioned differently when `backdrop-filter` is applied. If it is defined, `<section>` becomes a containing block and the `<h1>` is positioned relatively to it, without it, the `<h1>`  is positioned absolutely.

To authors this is an unexpected and undesired behavior. And it's unclear why this behavior is copied over from the `filter` property. For the `filter` property it might still be clear because it applies the filters to the element itself, though `backdrop-filter` filters what's behind the element, so this shouldn't have an effect on the layout of the element's children.

Sebastian

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Received on Tuesday, 1 December 2020 22:57:06 UTC