Spine actions UI broken in WP 5.6+

[admturner]

Description

Parts of the Spine actions (the search, contact, and share buttons) stop working in WP 5.6+. WordPress updated the bundled jQuery version to 3.5.1 in WP 5.6, but some parts of the Spine depend on syntax or features that were removed or deprecated in jQuery 3.5+.

Step-by-step reproduction instructions

1. Upgrade WordPress to 5.6+.
2. Verify the Enable jQuery Migrate Helper plugin is inactive.
3. Visit the site frontend.
4. Try selecting any of the Spine actions buttons, such as search.
5. Note that they don't respond.

WordPress information

• WordPress version: 5.6, 5.7.2, and 5.8-beta2

Device information

• Device: Desktop
• Operating system: Windows 10
• Browser: Firefox 89.0.1 and Chrome 91

[admturner]

Running the Enable jQuery Migrate Helper plugin provides a stopgap solution to this problem, but can raise others of its own and isn't intended as a permanent solution. I also ran into conflicts with other plugins that have upgraded to jQuery 3.5 plus without backwards compatibility with version 1 (TablePress is one that stopped working in jQuery < v3).