Qt: use the fusion style on Windows to work around wrong button label colors

Kind of unfortunate, but the `fusion` style looks nicer anyway.

Author: ClaudioHoffmann

examples/solum_qt/main.cpp

@@ -13,6 +13,24 @@ int main(int argc, char *argv[])
 {
     QApplication a(argc, argv);
 
+    // When using the default `windowsvista` style on Windows 10 in Dark Mode,
+    // Qt 6.5.1 renders the labels of disabled QPushButtons with the same
+    // black color as enabled ones, making it hard to distinguish the two:
+    //
+    //     https://bugreports.qt.io/browse/QTBUG-115036
+    //
+    // The `fusion` style does not have this issue, and looks much nicer in
+    // general. We only override the specific `windowsvista` style though:
+    // Other styles don't have this issue, and we do want to retain a
+    // potential system-wide Qt style setting, especially on Linux.
+    //
+    // QStyle::name() is also only available on Qt 6.1 and above.
+#if QT_VERSION >= QT_VERSION_CHECK(6, 1, 0)
+    if (a.style()->name() == "windowsvista") {
+        QApplication::setStyle("fusion");
+    }
+#endif
+
     QCoreApplication::setOrganizationName(QStringLiteral("Clarius"));
     QCoreApplication::setOrganizationDomain(QStringLiteral("clarius.com"));
     QCoreApplication::setApplicationName(QStringLiteral("Solum Demo"));