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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: Tab Atkins Jr. via GitHub <sysbot+gh@w3.org>
Date: Mon, 26 Oct 2020 21:52:16 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-716844351-1603749135-sysbot+gh@w3.org>
I also just realized the way I stated my conditions would imply that a 0x0 viewport and a nonzero-area element, with the element fully covering the viewport, would be considered "off-screen".

The intuition I'm actually going for, and should state more explicitly, is:
* if both shapes have non-zero area, their intersection must have non-zero area.
* if one or both shapes have zero area, they must just be coincident at at least one point.

This implies that two crossing lines are intersecting, that a point on the edge of a nonzero area is intersecting, etc. But two rectangles that are next to each other, sharing an edge, do not intersect.

Stated slightly differently, the justification for this intuition is that it's what you get if you forced a zero-area shape to have nonzero area by spreading it equally in all directions by an epsilon, but left shapes that already had nonzero area alone. (aka the "0x0 is actually a really tiny square" rule that we discussed during last week's meetings ^_^)

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