diff --git a/lib/__tests__/shortcut-alternatives.test.ts b/lib/__tests__/shortcut-alternatives.test.ts
new file mode 100644
index 000000000..73d202122
--- /dev/null
+++ b/lib/__tests__/shortcut-alternatives.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { SHORTCUT_APPS } from '@/lib/keyboard-shortcuts';
+import { shortcutAlternatives } from '@/lib/shortcut-keys';
+
+// The shortcuts help shows " / " as "or" between whole keys. "Shift + ↓ / ↑"
+// read as "Shift + ↓ or ↑", which says plain ↑ also extends the selection.
+
+describe('shortcut alternatives', () => {
+  it('spell out the modifier in every alternative', () => {
+    const bad: string[] = [];
+    for (const app of SHORTCUT_APPS) {
+      for (const section of app.sections) {
+        for (const entry of section.entries) {
+          const alternatives = shortcutAlternatives(entry.key);
+          const modified = alternatives.some((key) => key.includes(' + '));
+          if (modified && alternatives.some((key) => !key.includes(' + '))) bad.push(`${app.id}: ${entry.key}`);
+        }
+      }
+    }
+    expect(bad).toEqual([]);
+  });
+});
diff --git a/lib/keyboard-shortcuts/mail.ts b/lib/keyboard-shortcuts/mail.ts
index 1ee3eec54..639e5b179 100644
--- a/lib/keyboard-shortcuts/mail.ts
+++ b/lib/keyboard-shortcuts/mail.ts
@@ -27,7 +27,7 @@ export const MAIL_SHORTCUTS = {
     { key: "/", description: "shortcuts.global.search" },
     { key: "Shift + G", description: "shortcuts.global.refresh" },
     { key: "Mod + A", description: "shortcuts.global.select_all" },
-    { key: "Shift + ↓ / ↑", description: "shortcuts.global.extend_selection" },
+    { key: "Shift + ↓ / Shift + ↑", description: "shortcuts.global.extend_selection" },
   ],
   threads: [
     { key: "x", description: "shortcuts.threads.expand_collapse", layout: "default" },