[css-position] Define that containing block for fixed positioning is generated by the padding area

[Loirooriol]

https://drafts.csswg.org/css-position-3/#def-cb just says

If the element has position: fixed, the containing block is established by the viewport in the case of continuous media or the page area in the case of paged media.

However, the containing block can also be generated by an ancestor element e.g. with transform: scale(1).

In this case, the spec needs to define which area of the element (border area, padding area, content area) is used for generating the containing block.

Firefox, Chromium and WebKit(*) use the padding area, just like with absolute positioning. The spec should define this.

(*) WebKit uses the padding area for rendering, but the border area for getComputedStyle, https://bugs.webkit.org/show_bug.cgi?id=189518

[fantasai]

This is now fixed.

[Loirooriol]

Fixed by cc84557