Cover the re-inflation entry point with an editor.loadHTML security test

Add a system test group that drives the real production entry point
(editor.loadHTML) rather than the sanitizer in isolation: it re-parses
stored HTML under SAFE_FOR_XML and renders it into the live editor,
including attachment content re-parsed by AttachmentView. It asserts the
security invariant — no live event-handler attribute, no <script>, and no
execution — which is a browser-independent DOMPurify guarantee and so
holds across the Sauce matrix.

Author: jeremy

src/test/system.js

@@ -18,5 +18,6 @@ import "test/system/list_formatting_test"
 import "test/system/morphing_test"
 import "test/system/mutation_input_test"
 import "test/system/pasting_test"
+import "test/system/reinflation_security_test"
 import "test/system/text_formatting_test"
 import "test/system/undo_test"

src/test/system/reinflation_security_test.js

@@ -0,0 +1,46 @@
+import { assert, test, testGroup } from "test/test_helper"
+
+import { delay } from "../test_helpers/timing_helpers"
+
+// getEditorElement is installed as a global test helper (see trix/core/helpers/global),
+// mirroring how the other system tests reach the live editor.
+
+// Exercises the real re-inflation entry point rather than the sanitizer in isolation:
+// editor.loadHTML re-parses stored HTML under DOMPurify's mXSS-safe mode
+// (SAFE_FOR_XML: true) and renders it into the live editor, including any attachment
+// content re-parsed by AttachmentView. These assertions target the security invariant —
+// no executable handler and no <script> reach the live DOM, and nothing executes.
+// Handler stripping is a browser-independent DOMPurify guarantee, so unlike element
+// shape the assertions are parser-agnostic.
+testGroup("Re-inflation security (editor.loadHTML)", { template: "editor_empty" }, () => {
+  const loadAndAssertInert = async (html) => {
+    window.reinflationXSS = 0
+    getEditorElement().editor.loadHTML(html)
+    await delay(20)
+
+    const element = getEditorElement()
+    assert.equal(
+      element.querySelectorAll("[onerror], [onload], [onclick]").length, 0,
+      `live event handler survived re-inflation: ${element.innerHTML}`
+    )
+    assert.notOk(element.querySelector("script"), `script survived re-inflation: ${element.innerHTML}`)
+    assert.equal(window.reinflationXSS, 0, "re-inflated payload executed")
+
+    delete window.reinflationXSS
+  }
+
+  test("neutralizes a mutation-XSS payload loaded through the editor", async () => {
+    await loadAndAssertInert(
+      "<noscript><p title=\"</noscript><img src=x onerror=window.reinflationXSS=(window.reinflationXSS||0)+1>\">"
+    )
+  })
+
+  test("sanitizes attacker-controlled attachment content on re-inflation", async () => {
+    const attachment = {
+      contentType: "text/html5",
+      content: "</style><img src=x onerror=window.reinflationXSS=(window.reinflationXSS||0)+1>HELLO",
+    }
+    const html = `<div data-trix-attachment='${JSON.stringify(attachment).replace(/'/g, "&#39;")}'></div>`
+    await loadAndAssertInert(html)
+  })
+})