VR keyboard F-keys: F1-F12 in the suggestion strip while AltGr is active

keyboard.vr.functionKeys.enable (default off): while AltGr (Fn on layouts
without AltGr) is one-shot, locked or held, the strip shows F1-F12 instead
of the suggestions. A tap types the extra key VKX_F<n>, which the extra
keys' patch presses with xdotool together with the active Ctrl/Alt/Shift
(Alt+F4, Ctrl+F5) and releases the toggles; the text model resets, like
for Esc. The suggestions are kept as they are meanwhile, so when AltGr goes
off the strip shows the same items and selection again.

- vr-keyboard/function-keys.js: what the strip shows, the F-key keys.
- patch.js (VERSION 27): the strip's view, F-key picks, AltGr changes via
  a componentDidUpdate on the keyboard instance (and the poll).
- Allowlist: F1-F12 with any modifiers; Enter still refused.
- relay.mjs: up to 12 strip items.
- Assertions: needs keyboard.vr.enable, extraKeys.enable and a strip
  position "above"/"below" ("inside" would hide the AltGr number row).
- Tests: function-keys.test.mjs (switching and restoring, keys against
  the allowlist, text model), allowlist F-key cases.

modules/vr-keyboard/function-keys.js

@@ -0,0 +1,38 @@
+// function-keys.js: F1-F12 in the suggestion strip while AltGr is active
+// (keyboard.vr.functionKeys; patch.js). Evaluates to
+// { VERSION, KEYS, active, view, keyOf }.
+//
+// AltGr is Steam's toggle key (on layouts without one, keyboard.vr.extraKeys
+// adds it labelled Fn). Its state has three bits: 1 one-shot (released by the
+// next typed key), 2 locked (tapped twice), 4 held. Any of them shows the
+// F-keys, so a tapped AltGr shows them until the next key or until it's
+// tapped off; a held one while held.
+//
+// The F-keys only replace what the strip shows: the suggestions (patch.js
+// `cur`) are neither changed nor recomputed meanwhile, so when AltGr goes off
+// the strip shows the same items with the same selection again. An F-key
+// is typed as Steam key VKX_F<n>, an extra key: keyboard.vr.extraKeys presses
+// it with xdotool together with the active Ctrl/Alt/Shift and releases the
+// one-shot toggles, and patch.js resets its text model for it, like for Esc.
+(() => {
+  const VERSION = 1;
+  const KEYS = Array.from({ length: 12 }, (_, i) => `F${i + 1}`);
+  const FKEYS = { kind: 'fkeys' };                 // the source of an F-key view
+
+  const active = (toggleStates) => ((toggleStates?.AltGr ?? 0) & 7) !== 0;
+
+  // What the strip shows: { kind, items, index (the selected item, -1: none),
+  // source (the suggestion object itself, or the F-key marker) } or null.
+  // fnKeys: AltGr active and the keys can be sent; suggestions: patch.js
+  // `cur` if it's shown ({ items, index }), else null.
+  function view({ fnKeys, suggestions }) {
+    if (fnKeys) return { kind: 'fkeys', items: KEYS, index: -1, source: FKEYS };
+    if (suggestions) return { kind: 'suggestions', items: suggestions.items, index: suggestions.index, source: suggestions };
+    return null;
+  }
+
+  // The Steam key of F-key button i, or null.
+  const keyOf = (i) => (Number.isInteger(i) && KEYS[i] ? `VKX_${KEYS[i]}` : null);
+
+  return { VERSION, KEYS, active, view, keyOf };
+})()