Escape angle brackets in attachment JSON so pasted attachments survive DOMPurify

[jeremy]

Stacked on #1337 (base: fix-data-trix-attributes-safe-for-xml). This diff is only the layer on top; merge #1337 first.

The bug

Copying an attachment out of Trix and pasting it back silently dropped it whenever the attachment JSON contained </style> or another sequence DOMPurify's SAFE_FOR_XML mode treats as a raw-text or comment terminator. Paste runs the clipboard's text/html through HTMLParser under SAFE_FOR_XML, and DOMPurify's attribute rule removes the whole data-trix-attachment attribute on a match, before the forceKeepAttr set by Trix's uponSanitizeAttribute hook is honored. Quoted mail with an embedded <style> block is the common case: in HEY, 67 of 153 real mail bodies lost their embedded content on paste. Dragging the same content was lossless.

What #1337 does, and what this adds

#1337 stashes every data-trix-* value in uponSanitizeAttribute and puts it back in afterSanitizeAttributes once DOMPurify has dropped it. That stops the loss at the sanitizer for every data-trix-* attribute, without changing the HTML Trix emits.

This PR removes the trigger from the value itself, so nothing has to be put back:

• Emitting side. AttachmentView writes data-trix-attachment and data-trix-attributes with < and > escaped as < / >. Trix's own HTML then never carries a SAFE_FOR_XML trigger, so it survives a paste into any Trix build — including releases without either fix — and into anything else that runs DOMPurify in that mode.
• Sanitizing side. HTMLSanitizer rewrites those two attributes the same way before DOMPurify runs, so stored, server-rendered and older-Trix HTML with literal brackets is safe by construction: the rule has nothing to match and the restore in Pasting an embedded-content attachment may destroy it #1337 never has to fire for them.

The rewrite is lossless: in JSON text, angle brackets only occur inside string literals, where the escapes spell the same characters, so JSON.parse reads back the same value. Only values that already parse as JSON are rewritten; a malformed value is left alone, since HTMLParser ignores it either way.

Why both layers belong: #1337 is the general control — it covers every data-trix-* attribute, including ones that don't hold JSON — and this PR makes the two JSON attributes safe before that control is needed, on both ends of the pipe, so correctness doesn't rest on DOMPurify's hook ordering or on the consumer having the hook at all.

What the rebase changed

• The two CI commits this branch carried (Node 24 for the downstream Rails job, Ferrum process_timeout) are byte-identical to the ones in Pasting an embedded-content attachment may destroy it #1337 and dropped out of the rebase; Pasting an embedded-content attachment may destroy it #1337 is green with them, so there is nothing to split into a separate PR.
• Pasting an embedded-content attachment may destroy it #1337's keeps Trix attributes containing markup when sanitizing for XML now asserts parse-equality plus no raw angle brackets instead of byte-identity, because the value comes back escaped under this layer. Its other tests (html_parser_test, pasting_test) pass unchanged.
• Comments in html_sanitizer.js and its test describe the composition rather than each fix on its own.

Proof

• helpers/strings_test: escapes every bracket, round-trips through JSON.parse (backslashes before brackets, pre-escaped input, surrogate pairs, nesting), idempotent.
• html_sanitizer_test: attachment and caption JSON containing each of the twelve sequences in DOMPurify's regex survive under SAFE_FOR_XML with no raw brackets; a </style> attachment nested inside another attachment's content survives; malformed JSON is untouched.
• html_parser_test: the pasted markup parses back to an attachment with the original content and caption, including the nested case.
• document_view_test: rendered attachment JSON carries no raw brackets and parses back to the original.
• pasting_test (system): pasting the editor's own value for a <style>-bearing attachment with a </style> caption yields a second, identical attachment; the same for stored-shape markup with literal brackets.
• Each half is pinned independently: reverting attachment_view.js alone fails the render tests, reverting html_sanitizer.js alone fails the sanitizer and parser tests.
• Full suite locally on the stacked branch: 529 tests, 500 passed, 0 failed, 29 skipped.

[Copilot]

Pull request overview

Escapes angle brackets in attachment JSON before rendering and sanitization, preventing DOMPurify from dropping attachments during paste.

Changes:

• Added lossless JSON angle-bracket escaping.
• Applied escaping during attachment rendering and pre-sanitization.
• Added comprehensive unit and system regression coverage.

Tip

If you aren't ready for review, convert to a draft PR.
Click "Convert to draft" or run gh pr ready --undo.
Click "Ready for review" or run gh pr ready to reengage.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/trix/views/attachment_view.js	Escapes emitted attachment JSON attributes.
src/trix/models/html_sanitizer.js	Escapes valid legacy JSON before DOMPurify.
src/trix/core/helpers/strings.js	Adds the escaping helper.
src/test/unit/html_sanitizer_test.js	Covers DOMPurify trigger sequences.
src/test/unit/html_parser_test.js	Tests attachment parsing during paste.
src/test/unit/helpers/strings_test.js	Tests escaping and round-trip behavior.
src/test/unit/document_view_test.js	Verifies safely rendered attributes.
src/test/unit.js	Registers the new helper tests.
src/test/test_helpers/fixtures/fixtures.js	Updates expected attachment markup.
src/test/test_helpers/editor_helpers.js	Adds stored attachment HTML generation.
src/test/system/pasting_test.js	Adds end-to-end paste regressions.
action_text-trix/app/assets/javascripts/trix.js	Updates the generated browser bundle.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated no new comments.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-30T05:44:19.511796Z	8dd9a1f	New commits

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 62eab4c646

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".