[amp-lightbox-gallery] upgrade srcset on open or zoom to handle thumbnail usecase

[cathyxz]

To handle the use case of having small thumbnails show a high resolution image in lightbox gallery, we should try to upgrade the srcset / update sizes after opening the image in lightbox gallery. Some experimentation can be done with changing the sizes attribute and seeing whether browsers are able to handle this natively. Otherwise, we can revisit the logic to parse and find the correct src using srcset and upgrade on zoom. Blocked by #11575.

Here's a test case: https://codepen.io/cathyxz/full/rreWev/

[ampprojectbot]

This issue hasn't been updated in awhile. @cathyxz Do you have any updates?

[cathyxz]

See description edit for updates, in short we can resume this after #11575.

[ampprojectbot]

This issue hasn't been updated in awhile. @cathyxz Do you have any updates?

[cathyxz]

Already done via

amphtml/extensions/amp-image-viewer/0.1/amp-image-viewer.js

Lines 371 to 387 in 36fe72c

	updateSrc_() {
	if (!this.srcset_) {
	return Promise.resolve();
	}
	this.maxSeenScale_ = Math.max(this.maxSeenScale_, this.scale_);
	const width = Math.max(
	this.imageBox_.width * this.maxSeenScale_,
	this.sourceWidth_
	);
	const src = this.srcset_.select(width, this.getDpr());
	if (src == this.image_.getAttribute('src')) {
	return Promise.resolve();
	}
	return this.mutateElement(() => {
	this.image_.setAttribute('src', src);
	}, this.image_);
	}

, upgrades on zoom within the example.