Wide images are overflowing with <amp-img> but not with non-AMP <img>

[hellofromtonya]

What's the issue?

Wide <amp-img> images overflow the content area when its parent element has a width of fit-content. Behavior is consistent with fixed, intrinsic, and responsive layouts.

The same images in non-AMP <img> do not overflow the content area.

How do we reproduce the issue?

I've set up a demonstration of the AMP behavior compared to the same non-AMP images

1. Set the primary content area's width to less than target image's width.
2. Add a <figure> parent around the <amp-img> and applying fit-content as its width.

<figure class="wp-block-image">

where the CSS ruleset is:

.wp-block-image {
    width: -webkit-fit-content;
    width: -moz-fit-content;
    width: fit-content;
}

3. Set the image's src, width, height, srcset, and sizes attributes to a size wider than the content area:

For example, here is the <img> HTML:

<figure class="wp-block-image">
    <img src="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg" alt="Image Alignment 580x300" width="580" height="300" class="alignleft size-full wp-image-423" data-amp-layout="intrinsic" srcset="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg 580w, https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300-300x155.jpg 300w" sizes="(max-width: 580px) 100vw, 580px">
</figure>

Here is the AMP HTML:

<figure class="wp-block-image">
    <amp-img src="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg" alt="Image Alignment 580x300" width="580" height="300" class="alignleft size-full wp-image-423 amp-wp-enforced-sizes i-amphtml-element i-amphtml-layout-intrinsic i-amphtml-layout-size-defined i-amphtml-layout" layout="intrinsic" srcset="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg 580w, https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300-300x155.jpg 300w" sizes="(max-width: 580px) 100vw, 580px" style="width: 580px;">
        <i-amphtml-sizer class="i-amphtml-sizer">
            <img class="i-amphtml-intrinsic-sizer" src="data:image/svg+xml;charset=utf-8,&lt;svg height=&quot;300px&quot; width=&quot;580px&quot; xmlns=&quot;http://www.w3.org/2000/svg&quot; version=&quot;1.1&quot;/&gt;">
        </i-amphtml-sizer>
        <img decoding="async" alt="Image Alignment 580x300" srcset="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg 580w, https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300-300x155.jpg 300w" src="https://paired-ampconfdemo.pantheonsite.io/wp-content/uploads/2018/07/image-alignment-580x300.jpg" sizes="(max-width: 580px) 100vw, 580px" class="i-amphtml-fill-content i-amphtml-replaced-content">
    </amp-img>
</figure>

4. Expected: The image should proportionally scale to fit within the content area as demonstrated here

Please note, the content space is highlighted by the striped background to provide a quick glance of the image's relationship to the space.

5. Actual: The image overflows the content area as demonstrated here:

This issue may be related to Issue #17053.

What browsers are affected?

Chrome - Version 67.0.3396.99 (Official Build) (64-bit)
Firefox Quantum - 61.0.1 (64-bit)
Firefox Developer Edition - 62.0b11 (64-bit)
Safari Version 11.1.2 (13605.3.8)

MacOS Version 10.13.5
MacOS Version 10.13.6
Did not test on Windows.

Which AMP version is affected?

1531800879103

We noticed this issue on April 19, 2018 per amp-wp/1086.

[dreamofabear]

/to @aghassemi

[aghassemi]

related to #17053
NOT a regression of srcset change. (I don't think #17053 is either)
/cc @cathyxz FYI for now. will create a mini project for this and related issues to find the best solution and collaborate with @hellofromtonya and WP to ensure the solution is correct based on expectations.

[ampprojectbot]

This issue seems to be in Pending Triage for awhile. @aghassemi Please triage this to an appropriate milestone.

[cathyxz]

I think we triaged this along with #17053 to be due to the sizes attribute having unexpected side effects. I've tried to enable sizes without side-effects by autogenerating sizes in #20968, and am now updating the <amp-img> documentation to better reflect our sizes behavior in AMP. I think we should be able to close this issue with #22053.

[westonruter]

Yes, I think this is resolved with amp-img-auto-sizes.

[hellofromtonya]

Yes, it is resolved.