Themes: Lazy load screenshots

[mmtr]

Fixes #95248

Proposed Changes

Defers the loading of the theme screenshots in the theme showcase until the user scrolls near them with a loading='lazy' attribute, effectively decreasing the initial data load from 35MB to 2.5MB (-92%).

Before

Screen.Recording.2024-12-02.at.11.11.31.mov

After

Screen.Recording.2024-12-02.at.11.24.57.mov

Why are these changes being made?

To optimize the data transferred over network.

Testing Instructions

• Use the Calypso live link below
• Open the browser devtools and select the Network tab
• Filter by screenshot requests
• Go to /themes/:site
• Make sure that the screenshot requests are lazy loaded (off-screen images are loaded when you scroll near them)
• Repeat with /themes while logged in and logged out

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126134

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126134&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126134&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/lazy-load-theme-screenshots on your sandbox.

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~3 bytes added 📈 [gzipped])

name    parsed_size           gzip_size
themes        +15 B  (+0.0%)       +3 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~3 bytes added 📈 [gzipped])

name                                     parsed_size           gzip_size
async-load-signup-steps-theme-selection        +15 B  (+0.0%)       +3 B  (+0.0%)
async-load-design-blocks                       +15 B  (+0.0%)       +3 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.