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[css-contain] content-visibility: auto and SVG-as-image #10347

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emilio opened this issue May 17, 2024 · 9 comments
Open

[css-contain] content-visibility: auto and SVG-as-image #10347

emilio opened this issue May 17, 2024 · 9 comments
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Agenda+ css-contain-2 Current Work

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@emilio
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emilio commented May 17, 2024

Over in Mozilla bug 1894546, we got a report about something like an (invisible) SVG image rendering content-visibility: auto elements in Chromium but not Firefox / Safari.

Off-hand, I think per spec it is invalid. At least, I don't see how the viewport checks would run, since they run from the "update the rendering" steps in HTML.

I think that bug is invalid, but seems worth double-checking... @vmpstr do you know what's going on in Chromium? Should I file a Blink bug? Have I missed something that should make that work per spec?
@vmpstr
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vmpstr commented May 21, 2024

Doesn't the SVG width and height define a viewport for content-visibility within that image?

This is essentially what Chromium does: anytime we paint an SVG, we set a "viewport" on it into which we paint and update all of its rendering as if that was the viewport (including intersection observer determination for c-v).

/cc @progers

@emilio
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emilio commented May 21, 2024

Hmm... does chromium run the whole lifecycle update for SVG images? That seems generally wasteful? Also, what if the image is out of view in the parent document? That'd behave inconsistently with <iframe> unless I'm missing something.

@vmpstr
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vmpstr commented May 22, 2024

I believe Chromium does that yeah (@progers would know for sure). I suspect it's exactly to support cases like this. We're drawing this SVG into a canvas, and then using the pixels for some other reason. It's very disconnected from when we actually use the pixels and presumably we want to treat those pixels as if they were visible when painted.

content-visibility aside, Chromium does it also because of paint culling that we do for offscreen content. We basically want to ensure when we draw the SVG into a canvas in this way, the culling is not cutting off any paint.

Can you elaborate on the iframe inconsistency, I'm not sure I follow

@progers
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progers commented May 22, 2024

does chromium run the whole lifecycle update for SVG images?

Chrome does run most of the steps for a full animation frame (see: SVGImage::PaintRecordForCurrentFrame), but steps like resize observer are no-ops. Chrome isn't calculating real user viewport intersection for images though, and is just treating the image contents as relevant to the user, regardless of whether the image is on screen.

Offscreen content brought on-screen via a filter is considered relevant to the user (spec). I think ctx.drawImage should work similarly.

@emilio
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emilio commented May 22, 2024

I mean, at least Gecko shares SVG-as-images.

A content-visibility: auto element in an iframe will, IIRC use an IntersectionObserver-like check to intersect with the top level viewport. But that doesn't work if your image can be painted in multiple places and the data is shared.

Consider:

<!doctype html>
<img src="foo.svg">
<div style="height: 5000px"></div>
<img src="foo.svg">

IIUC, the content would be "relevant" in both images regardless. While if they were iframes, the content will be relevant in the top iframe, but not in the bottom.

So I think that, effectively for this purpose at least, you just ignore content-visibility: auto (everything is relevant in an SVG image, kind of)

@progers
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progers commented May 22, 2024

I mean, at least Gecko shares SVG-as-images.

Chrome does this as well. We need to pass in some context-specific data when drawing the image, such as when two <img>s reference the same svg but have different sizes (code). It would be technically possible to include top-level viewport intersection in this context-specific data.

There are are some spec'd differences between svgs in iframes vs svg images, such as how svg image do not allow interaction or script (secure animated mode). Because of the restrictions of svg images, there is almost no observable difference if we just consider them as always relevant to the user vs actually calculating top-level-viewport intersection.

@progers
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progers commented May 22, 2024

almost

After the initial, synchronous, viewport-proximity check in update-the-rendering (checkForInitialDetermination), subsequent viewport-proximity is updated in the next rendering opportunity, and so is one frame late. Animations in svg could be used with this one-frame-late aspect to show a user-visible difference between svg-iframe and svg-image content-visibility behavior. Because of this, and ctx.drawImage, I think it would be useful to specify whether svg-as-image is always relevant to the user (I am in favor).

@emilio
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emilio commented May 22, 2024

That seems reasonable. Honestly I am a bit surprised about this use case as a whole, but that is trivial to implement and easy to test too.

@vmpstr
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vmpstr commented May 22, 2024

I am also surprised about this case -- it took a big of digging to figure out what Blink was doing. I also support making svg-as-image cases always relevant

@Loirooriol Loirooriol added css-contain-2 Current Work and removed css-contain-1 labels May 27, 2024
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