Handling unidentified/death keys in the keyboard history UI

Start at app/static/js/app.js line 72, where the keyboard history relies on KeyboardEvent.key. Reproduce the alt+L case with the documented MacOS layout across Chrome, Safari, and Firefox, then verify that handling for Unidentified and Dead keys produces consistent, useful history in the UI without changing backend or HID behavior.

I noticed one quirk in the UI, regarding the keypress history in the status bar. Note: this only affects the UI, not the backend or HID interface!

When the client machine is configured to use a keyboard layout that doesn’t provide a mapping for certain keys or key combinations, the keyboard history shows inconsistent results across browsers. From the view point of the target machine, however, things work as expected. So this issue is specific to using non-EN-US keyboard layouts.

I tested this on a MacOS machine with a customised keyboard layout, which doesn’t define a mapping for the alt+L key combination. So pressing alt+L on the client machine wouldn’t produce any character on the client machine itself. However, on the target machine, the alt+L combination may be well-defined, e.g. if the target machine uses a German keyboard layout where alt+L maps to the @ character. In this scenario, TinyPilot would forward the alt+L combination to the target machine, and produce the @ there.

In the TinyPilot’s web UI, however, I see these results in the status bar:

• Chrome displays: alt+Unidentified (note: it’s not undefined!)
  • Screenshot
  • KeyboardEvent
• Safari displays: alt+Dead
  • Screenshot
  • KeyboardEvent
• Firefox displays: alt+l (as I would expect)
  • Screenshot
  • KeyboardEvent

As stated in the documentation, the key property of the KeyboardEvent object may be Unidentified or Dead:

• If the KeyboardEvent represents the press of a dead key, the key value must be "Dead".
• If the key cannot be identified, the returned value is Unidentified.

For the keyboard history, we rely on that key property, which is why we see this.