Write: Keep selected text highlighted while the link popover is open

[allilevine]

Fixes RSM-1599

Proposed changes

• Use the CSS Custom Highlight API to visually highlight selected text while the link popover is open, so users can see which text will receive the link even though focus moves to the URL input.
• When the cursor is collapsed inside an existing link and the link popover is opened, auto-select the full link text and highlight it.
• Clear the highlight in all close paths (Apply, Remove, Enter, Escape, outside click, toggle off).

link.highlighting.mov

Related product discussion/links

• RSM-1599

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Go to the Write editor (/wp-admin/admin.php?page=write)
• Type some text (e.g. "Hello world foo bar")
• Select a portion of text (e.g. "world foo")
• Click the Link button in the toolbar
• Verify: the selected text stays visually highlighted with a blue background while the URL input is focused. It's intentionally slightly different from the selection background (which we can't match exactly with this api).
• Type a URL and press Enter — verify the link is applied to the correct text
• Click inside the linked text, then click the Link button again
• Verify: the full link text is highlighted and the URL is pre-filled
• Press Escape — verify the highlight is removed
• Repeat and click outside the popover — verify the highlight is removed
• Test with bold/italic text selections to ensure cross-boundary selections work

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (WordPress.com Site Helper), and enable the fix/write-link-selection-highlight branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/write-link-selection-highlight

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[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report

[kbrown9]

This tested well for me, and the code changes look good.

I did run into a pre-existing issue where closing the link popover with Escape or an outside click sets the cursor to the beginning of the content. I'll open a Linear issue for that.