fix(web-shell): add explicit ::selection for message content in Firefox

packages/web-shell/client/e2e/web-shell.smoke.spec.ts

@@ -40,6 +40,19 @@ test('loads replayed transcript and connects to fake daemon @smoke', async ({
   await expect(page.locator('[data-web-shell-message-list]')).toContainText(
     'Hello from fake daemon',
   );
+
+  // #8214: pin the explicit ::selection rule on message content. This
+  // asserts the rule is present and matches the [data-user-selectable]
+  // wrapper; it does not verify the Firefox paint effect itself (this
+  // repo's Playwright projects are chromium-only).
+  const selectionBackground = await page.evaluate(() => {
+    const content = document.querySelector(
+      '[data-user-selectable] *',
+    ) as Element | null;
+    if (!content) return null;
+    return getComputedStyle(content, '::selection').backgroundColor;
+  });
+  expect(selectionBackground).toBe('rgba(0, 128, 255, 0.3)');
 });
 
 test('submits a prompt and renders a streamed assistant response @smoke', async ({

packages/web-shell/client/styles/standalone.css

@@ -101,6 +101,23 @@ textarea,
   user-select: text;
 }
 
+/*
+ * Defensive selection-highlight rule for message content. #8214.
+ *
+ * Firefox is suspected of not painting the default selection highlight for
+ * text whose element chain passes through a `display: contents` element (the
+ * `data-user-selectable` wrapper on MessageItem). The logical selection
+ * (copy, selectionchange popup) works fine - only the visual highlight is
+ * missing. An explicit `::selection` background makes Firefox paint the
+ * highlight where the default painting may fail. This is a low-risk
+ * workaround, not a confirmed root-cause fix: the wrapper is shared by user
+ * and assistant rows alike, and the reporter's environment appears to be an
+ * embedding page, so its own CSS may also need adjustment.
+ */
+[data-user-selectable] ::selection {
+  background: hsl(210 100% 50% / 30%);
+}
+
 /*
  * iOS Safari auto-zooms any <input> / <textarea> / contenteditable whose
  * computed font-size is < 16px when focused. The composer editor sits at