Draft: Lazy Image Loading

[kean]

In the current implementation, including the production version, when the comment that has media is loading, you can see part of the text but the height of the cell isn't updated until the image is loaded.

before.mov

It's possible to update the height dynamically as the page is loading by observing webKit.scrollView.contentSize. For best use experience, it should also show some kind of placeholders during loading.

This implementation should also enable correct rendering of features like dropdown or excerpts in comments that add/remove content from the "page". Unfortunately, I couldn't quite get it to work reliably with the WKWebView reuse in place. This PR covered WIP.

after.mov

Notes

• func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) gets called when all image are loaded
• webView.scrollView.contentSize reports incorrect size that almost never quite matches the scroll height of the page
• ReaderDetailsViewController implements something similar but it doesn't have the same constraints: reuse of WKWebView.

To test:

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

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