DolphinQt: Make Windows tab bar scroll buttons opaque

Source/Core/DolphinQt/Settings.cpp

@@ -13,6 +13,15 @@
 #include <QSize>
 #include <QWidget>
 
+#ifdef _WIN32
+#include <memory>
+
+#include <fmt/format.h>
+
+#include <QTabBar>
+#include <QToolButton>
+#endif
+
 #include "AudioCommon/AudioCommon.h"
 
 #include "Common/Config/Config.h"
@@ -146,6 +155,22 @@ void Settings::SetCurrentUserStyle(const QString& stylesheet_name)
             .arg(border_color.rgba(), 0, 16);
     stylesheet_contents.append(QStringLiteral("%1").arg(tooltip_stylesheet));
   }
+#ifdef _WIN32
+  // MSVC has a bug causing QTabBar scroll buttons to be partially transparent when they inherit any
+  // stylesheet (see https://bugreports.qt.io/browse/QTBUG-74187) which is triggered when setting
+  // qApp's stylesheet below. Setting the scroll buttons' color directly fixes the problem.
+
+  // Create a temporary QToolButton that's a child of a QTabBar in case that has different styling
+  // than a plain QToolButton.
+  const auto tab_bar = std::make_unique<QTabBar>();
+  auto* const tool_button = new QToolButton(tab_bar.get());
+
+  const QRgb background_color = tool_button->palette().color(QPalette::Button).rgba();
+
+  const std::string style_var =
+      fmt::format("QTabBar QToolButton {{ background-color: #{:08x}; }}", background_color);
+  stylesheet_contents.append(QString::fromStdString(style_var));
+#endif
 
   qApp->setStyleSheet(stylesheet_contents);