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Re: [csswg-drafts] [css-contain-2] Spec is unclear about whether 0-sized boxes can be "relevant to the user" (since they have zero intersection area) (#5641)

From: Daniel Holbert via GitHub <sysbot+gh@w3.org>
Date: Mon, 26 Oct 2020 05:02:10 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-716304274-1603688529-sysbot+gh@w3.org>
@Loirooriol Thanks for that scenario & graphic -- good food for thought. However, I think it's a flawed analogy, because:
 - the intersection between two planes (if it exists) is *always a line* -- that's what it's defined as, basically.
 - whereas, the intersection between two shapes in 2D space is defined as another 2D shape; and one (perhaps-naive) implementation of "do they intersect" is to find out whether that shape is empty.

Also, there are edge cases that make this somewhat more interesting. E.g. consider these cases (where "0-sized box" means height:0;width:0;)
 (1) Does a viewport-adjacent box (e.g. with `left-100px;width:100px`) count as intersecting the viewport? (I would assume "no"; none of it is onscreen).
 (2) Does a 0-sized box positioned at `1,1` count as intersecting the viewport? (I would assume "yes"; it's clearly on-screen, even if it doesn't render.)
 (3) Supposing the viewport itself were 0-sized (kind of extreme, but humor me): does a 0-sized box at 0,0 intersect the viewport in that case? (I would assume yes, since these are two coincident points, which "intersect" as much as points possibly can intersect each other.) 

 (4) Now the interesting part (I think): does a 0-sized box positioned at `0,0` count as intersecting the viewport? (This is unclear IMO; the logic of (1) suggests "no", whereas the logic of (2) and (3) suggests "yes".)

**I mostly want to make sure case (4) is well-defined**, since that's the one that feels ambiguous to me.  Depending on what the decision & justification is for case 4, that informs what the answers are for cases (1)-(3), too.

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