Hold Keys mode: keys stay down while touched, like a hardware keyboard

- HardwareKeys presses a key's unshifted keysym on touch and releases it
  on lift, through fake input; held keys repeat and combine as on a
  physical keyboard (games, Shift+click)
- Shift/Ctrl/Alt/Super are plain held keys, caps lock is the system's,
  dead keys compose in the app
- No long-press, flick, trackpad or rollover in this mode; nothing is
  left held when the mode is turned off or touches are cancelled
- Settings switch; default stays the touch (virtual keyboard) mode

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/input/KeyboardController.cpp

@@ -22,6 +22,7 @@ KeyboardController::KeyboardController(InputEngine &engine, LayoutModel &model,
     : QObject(parent)
     , m_engine(engine)
     , m_model(model)
+    , m_hardware(engine)
     , m_keysymRepeater(KeysymRepeat)
 {
     connect(&m_backspaceRepeater, &KeyRepeater::repeat, this, &KeyboardController::deleteBackward);
@@ -30,11 +31,34 @@ KeyboardController::KeyboardController(InputEngine &engine, LayoutModel &model,
 
 KeyboardController::ShiftMode KeyboardController::shiftMode() const
 {
+    if (m_options.holdKeys) {
+        if (m_hardware.isCapsLockOn())
+            return ShiftMode::Locked;
+        return m_hardware.isShiftHeld() ? ShiftMode::Once : ShiftMode::Off;
+    }
     return ShiftMode(m_shift.mode());
 }
 
+QVariantMap KeyboardController::modifierModes() const
+{
+    return m_options.holdKeys ? m_hardware.modifierModes() : m_modifiers.modes();
+}
+
+bool KeyboardController::holdKey(const KeyDef &key, bool pressed)
+{
+    if (!m_options.holdKeys || !HardwareKeys::handles(key))
+        return false;
+    pressed ? m_hardware.press(key) : m_hardware.release(key);
+    m_model.setShiftLevel(shiftLevel());
+    Q_EMIT shiftChanged();
+    Q_EMIT modifiersChanged();
+    return true;
+}
+
 void KeyboardController::keyDown(const KeyDef &key, qint64 timeMs)
 {
+    if (holdKey(key, true))
+        return;
     switch (key.type) {
     case KeyType::Shift:
         changeShift([&] { m_shift.press(timeMs); });
@@ -72,6 +96,8 @@ void KeyboardController::keyDown(const KeyDef &key, qint64 timeMs)
 
 void KeyboardController::keyUp(const KeyDef &key, KeyOutput output, const QString &alternate, qint64)
 {
+    if (holdKey(key, false))
+        return;
     switch (key.type) {
     case KeyType::Shift:
         changeShift([&] { m_shift.release(); });
@@ -112,6 +138,8 @@ void KeyboardController::keyUp(const KeyDef &key, KeyOutput output, const QStrin
 
 void KeyboardController::keyCancelled(const KeyDef &key)
 {
+    if (holdKey(key, false))
+        return;
     if (key.type == KeyType::Shift)
         changeShift([&] { m_shift.release(); });
     else if (key.type == KeyType::Modifier) {
@@ -125,6 +153,7 @@ void KeyboardController::keyCancelled(const KeyDef &key)
 
 void KeyboardController::slideReleased(const KeyDef &target, qint64 timeMs)
 {
+    holdKey(target, true); // only in hold-keys mode: the slide never pressed it
     keyUp(target, KeyOutput::Primary, {}, timeMs);
     m_model.setLayer(m_layerBeforeSwitch);
 }
@@ -158,7 +187,12 @@ void KeyboardController::setContext(const TextContext &context)
 
 void KeyboardController::setOptions(const TypingOptions &options)
 {
+    if (m_options.holdKeys && !options.holdKeys)
+        m_hardware.releaseAll();
     m_options = options;
+    m_model.setShiftLevel(shiftLevel());
+    Q_EMIT shiftChanged();
+    Q_EMIT modifiersChanged();
     autoCapitalize();
 }
 
@@ -254,6 +288,8 @@ void KeyboardController::autoCapitalize()
 
 ShiftLevel KeyboardController::shiftLevel() const
 {
+    if (m_options.holdKeys)
+        return m_hardware.shiftLevel();
     if (m_shift.appliesToShortcuts())
         return ShiftLevel::Shifted;
     return m_shift.isActive() ? ShiftLevel::Capitals : ShiftLevel::None;