fn Key does not work as shortcut

**Describe the bug**
the global fn Key on a Mac does not work as shortcut

**To Reproduce**
Steps to reproduce the behavior:
1. Go to Settings
2. Try to set the fn key as the trigger key

**Expected behavior**
stays empty

**Desktop (please complete the following information):**
- Version 0.6.10 (79)

Start in the macOS app's Settings flow and reproduce the issue by trying to set the fn key as the trigger key. Trace how the selected trigger key is captured and stored, then verify that choosing fn no longer leaves the setting empty.

Written by the indexing model from the issue text.