xkb: Explain mystery constant

While looking at xkb code I found what this + 8 meant, figured we better
include it in our code so it feels less magical.

src/xkb.cpp

@@ -30,6 +30,10 @@
 
 Q_LOGGING_CATEGORY(KWIN_XKB, "kwin_xkbcommon", QtWarningMsg)
 
+/* The offset between KEY_* numbering, and keycodes in the XKB evdev
+ * dataset. */
+static const int EVDEV_OFFSET = 8;
+
 namespace KWin
 {
 
@@ -353,7 +357,7 @@ void Xkb::updateKey(uint32_t key, InputRedirection::KeyboardKeyState state)
     if (!m_keymap || !m_state) {
         return;
     }
-    xkb_state_update_key(m_state, key + 8, static_cast<xkb_key_direction>(state));
+    xkb_state_update_key(m_state, key + EVDEV_OFFSET, static_cast<xkb_key_direction>(state));
     if (state == InputRedirection::KeyboardKeyPressed) {
         const auto sym = toKeysym(key);
         if (m_compose.state && xkb_compose_state_feed(m_compose.state, sym) == XKB_COMPOSE_FEED_ACCEPTED) {
@@ -459,16 +463,16 @@ QString Xkb::layoutShortName(int index) const
 void Xkb::updateConsumedModifiers(uint32_t key)
 {
     Qt::KeyboardModifiers mods = Qt::NoModifier;
-    if (xkb_state_mod_index_is_consumed2(m_state, key + 8, m_shiftModifier, XKB_CONSUMED_MODE_GTK) == 1) {
+    if (xkb_state_mod_index_is_consumed2(m_state, key + EVDEV_OFFSET, m_shiftModifier, XKB_CONSUMED_MODE_GTK) == 1) {
         mods |= Qt::ShiftModifier;
     }
-    if (xkb_state_mod_index_is_consumed2(m_state, key + 8, m_altModifier, XKB_CONSUMED_MODE_GTK) == 1) {
+    if (xkb_state_mod_index_is_consumed2(m_state, key + EVDEV_OFFSET, m_altModifier, XKB_CONSUMED_MODE_GTK) == 1) {
         mods |= Qt::AltModifier;
     }
-    if (xkb_state_mod_index_is_consumed2(m_state, key + 8, m_controlModifier, XKB_CONSUMED_MODE_GTK) == 1) {
+    if (xkb_state_mod_index_is_consumed2(m_state, key + EVDEV_OFFSET, m_controlModifier, XKB_CONSUMED_MODE_GTK) == 1) {
         mods |= Qt::ControlModifier;
     }
-    if (xkb_state_mod_index_is_consumed2(m_state, key + 8, m_metaModifier, XKB_CONSUMED_MODE_GTK) == 1) {
+    if (xkb_state_mod_index_is_consumed2(m_state, key + EVDEV_OFFSET, m_metaModifier, XKB_CONSUMED_MODE_GTK) == 1) {
         mods |= Qt::MetaModifier;
     }
     m_consumedModifiers = mods;
@@ -512,7 +516,7 @@ xkb_keysym_t Xkb::toKeysym(uint32_t key)
     if (!m_state) {
         return XKB_KEY_NoSymbol;
     }
-    return xkb_state_key_get_one_sym(m_state, key + 8);
+    return xkb_state_key_get_one_sym(m_state, key + EVDEV_OFFSET);
 }
 
 QString Xkb::toString(xkb_keysym_t keysym)
@@ -534,7 +538,7 @@ Qt::Key Xkb::toQtKey(xkb_keysym_t keySym,
                      bool superAsMeta) const
 {
     // FIXME: passing superAsMeta doesn't have impact due to bug in the Qt function, so handle it below
-    Qt::Key qtKey = Qt::Key(QXkbCommon::keysymToQtKey(keySym, modifiers, m_state, scanCode + 8, superAsMeta));
+    Qt::Key qtKey = Qt::Key(QXkbCommon::keysymToQtKey(keySym, modifiers, m_state, scanCode + EVDEV_OFFSET, superAsMeta));
 
     // FIXME: workarounds for symbols currently wrong/not mappable via keysymToQtKey()
     if (superAsMeta && (qtKey == Qt::Key_Super_L || qtKey == Qt::Key_Super_R)) {
@@ -557,7 +561,7 @@ bool Xkb::shouldKeyRepeat(quint32 key) const
     if (!m_keymap) {
         return false;
     }
-    return xkb_keymap_key_repeats(m_keymap, key + 8) != 0;
+    return xkb_keymap_key_repeats(m_keymap, key + EVDEV_OFFSET) != 0;
 }
 
 void Xkb::switchToNextLayout()