qxkbcommon: Amend comment for Qt::Key upper-case transformation

This should make it clear enough that the issue here is a quirk of
Qt::Key and not something otherwise special about case-change affecting
whether a character is in Latin-1 (for all that this is the root cause
of the quirk in Qt::Key).

Pick-to: 6.5
Change-Id: I80fa6b78a0d0c431401a3fea18edf6149de08b92
Reviewed-by: Andrey Butirsky <butirsky@gmail.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/platform/unix/qxkbcommon.cpp

@@ -510,13 +510,13 @@ static int keysymToQtKey_internal(xkb_keysym_t keysym, Qt::KeyboardModifiers mod
         // numeric keypad keys
         qtKey = Qt::Key_0 + (keysym - XKB_KEY_KP_0);
     } else if (QXkbCommon::isLatin1(keysym)) {
-        // Upper-case first, since Qt::Keys are defined in terms of their
-        // upper-case versions.
+        // Most Qt::Key values are determined by their upper-case version,
+        // where this is in the Latin-1 repertoire. So start with that:
         qtKey = QXkbCommon::qxkbcommon_xkb_keysym_to_upper(keysym);
-        // Upper-casing a Latin1 character might move it out of Latin1 range,
-        // for example U+00B5 MICRO SIGN, which upper-case equivalent is
-        // U+039C GREEK CAPITAL LETTER MU. If that's the case, then map the
-        // original lower-case character.
+        // However, Key_mu and Key_ydiaeresis are U+00B5 MICRO SIGN and
+        // U+00FF LATIN SMALL LETTER Y WITH DIAERESIS, both lower-case,
+        // with upper-case forms outside Latin-1, so use them as they are
+        // since they're the Qt::Key values.
         if (!QXkbCommon::isLatin1(qtKey))
             qtKey = keysym;
     } else {