Stacking order + 'z-index' spec

[fantasai]

Filing this issue to track that we need a stacking/painting order spec. Topics should include (note this list has been edited substantially since the issue was initially filed):

• stacking order (for painting and hit testing), with key additions from https://www.w3.org/TR/CSS2/zindex.html that should include:
  • column rules
  • all the effects of the exported terminology described below
  • Adjust z-index description to handle grid/flex which automatically apply it not just to positioned items.
• general painting model
• general hit testing model
• terminology that other specs can hook into to to describe:
  • properties that establish new stacking context
  • properties that establish containing blocks for absolute and fixed positioned elements (or just absolute, if needed)
  • properties that establish an isolated group (already defined to be equivalent to stacking context)
• anything that creates a stacking context should sort in the positioned descendants z-ordering list #2717
• [css-view-transitions-1] Where is “effects” defined? #9139 (comment)
• Maybe: rasterization and what a "raster pixel" is, for defining things such as border snapping and other effects. See discussion in [css-values] Should define the terms "device pixels" and "raster pixels" #7287.
• Maybe: general pixel snapping model for CSS rendering (generally snapping of edges after layout, but for some properties snapping of sizes before layout)

(Feel free to edit this description to add more tracking items, or comment below and I'll edit if you don't have permissions.)

[Loirooriol]

CSS2 seems to assume that stacking contexts can only be established by positioned elements with non-auto z-index (and the root element).

But now we have plenty of ways for that. Like the transform and opacity listed above, but also will-change, contain, isolation, filter, perspective, etc. The new text should not make assumptions about how the stacking context was established, since other specs can introduce new ways of establishing it.

[dbaron]

It should specify the resolution of #2717.

[dbaron]

Also, I'd probably name the spec "CSS Painting Model" or something along those lines. And it definitely needs to define terms for the various things that can happen (mentioned in the previous 2 comments) that other specs can hook into.

[BenjaminAster]

z-index is now the only CSS property that is not defined anywhere outside of the CSS2 spec, so I think creating a spec for it (or merging it into an existing one) would be a big step forward in making the CSS2 specification obsolete.

[Loirooriol]

Painting order and stacking contexts are now in https://drafts.csswg.org/css-position-4/#painting-order, so I guess z-index could be there too.

[tabatkins]

Oh yeah it should definitely live in Position.

[dbaron]

For now I think that's fine, though in the long term I think we should really have a CSS painting model spec (as discussed at least once in the past).

[dbaron]

I just edited the initial comment to include the other things that have been raised here, and in that other linked discussion. (The list more than doubled in length, I think.)