Keep the selection when a drag crosses a custom attachment

app/assets/stylesheets/lexxy-content.css

@@ -478,6 +478,17 @@
     position: relative;
     white-space: normal;
 
+    /* Gecko drops the whole selection when a drag crosses a contenteditable="false"
+       island unless it is selectable as a single unit. Editor-only: the rendered
+       view has no contenteditable and must keep normal text selection. */
+    lexxy-editor & {
+      -webkit-user-select: all;
+
+      @supports (user-select: all) {
+        user-select: all;
+      }
+    }
+
     img {
       block-size: var(--lexxy-attachment-image-size);
       border-radius: 50%;

test/browser/tests/prompts/selection_across_mention.test.js

@@ -0,0 +1,47 @@
+import { test } from "../../test_helper.js"
+import { expect } from "@playwright/test"
+
+const MENTION = '<action-text-attachment sgid="test-sgid-zacharias" content-type="application/vnd.actiontext.mention" content="&lt;span class=&quot;person person--inline&quot;&gt;Zacharias&lt;/span&gt;"></action-text-attachment>'
+
+async function dragAcrossParagraph(page, editor) {
+  const paragraph = await editor.content.locator("p").first().boundingBox()
+  const y = paragraph.y + paragraph.height / 2
+
+  await page.mouse.move(paragraph.x + 4, y)
+  await page.mouse.down()
+  await page.mouse.move(paragraph.x + paragraph.width - 4, y, { steps: 15 })
+  await page.mouse.up()
+  await editor.flush()
+}
+
+function selectedText(page) {
+  return page.evaluate(() => window.getSelection().toString())
+}
+
+test.describe("Selecting content that contains a mention", () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto("/mentions.html")
+    await page.waitForSelector("lexxy-editor[connected]")
+  })
+
+  test("dragging across a mention selects the surrounding text", async ({ page, editor }) => {
+    await editor.setValue(`<p>Hello ${MENTION} world</p>`)
+    await editor.flush()
+
+    await dragAcrossParagraph(page, editor)
+
+    const selection = await selectedText(page)
+    expect(selection).toContain("Hello")
+    expect(selection).toContain("world")
+    expect(selection).toContain("Zacharias")
+  })
+
+  test("dragging across plain text still selects it", async ({ page, editor }) => {
+    await editor.setValue("<p>Hello there world</p>")
+    await editor.flush()
+
+    await dragAcrossParagraph(page, editor)
+
+    expect(await selectedText(page)).toContain("Hello there world")
+  })
+})