Qt: allow to use native styles

rpcs3/rpcs3qt/gui_application.cpp

@@ -36,6 +36,7 @@
 #include <QFileInfo>
 #include <QMessageBox>
 #include <QTextDocument>
+#include <QStyleFactory>
 
 #include <clocale>
 
@@ -746,14 +747,55 @@ void gui_application::OnChangeStyleSheetRequest()
 
 	const QString stylesheet_name = m_gui_settings->GetValue(gui::m_currentStylesheet).toString();
 
+	// Reset style to default before doing anything else, or we will get unexpected effects in custom stylesheets.
+	if (const QStringList styles = QStyleFactory::keys(); !styles.empty())
+	{
+		// The first style is the default style according to the Qt docs.
+		if (QStyle* style = QStyleFactory::create(styles.front()))
+		{
+			setStyle(style);
+		}
+	}
+
+	const auto match_native_style = [&stylesheet_name]() -> QString
+	{
+		// Search for "native (<style>)"
+		static const QRegularExpression expr(gui::NativeStylesheet + " \\((?<style>.*)\\)");
+		const QRegularExpressionMatch match = expr.match(stylesheet_name);
+
+		if (match.hasMatch())
+		{
+			return match.captured("style");
+		}
+
+		return {};
+	};
+
+	gui_log.notice("Changing stylesheet to '%s'", stylesheet_name);
+
 	if (stylesheet_name.isEmpty() || stylesheet_name == gui::DefaultStylesheet)
 	{
+		gui_log.notice("Using default stylesheet");
 		setStyleSheet(gui::stylesheets::default_style_sheet);
 	}
 	else if (stylesheet_name == gui::NoStylesheet)
 	{
+		gui_log.notice("Using empty style");
 		setStyleSheet("/* none */");
 	}
+	else if (const QString native_style = match_native_style(); !native_style.isEmpty())
+	{
+		if (QStyle* style = QStyleFactory::create(native_style))
+		{
+			gui_log.notice("Using native style '%s'", native_style);
+			setStyleSheet("/* none */");
+			setStyle(style);
+		}
+		else
+		{
+			gui_log.error("Failed to set stylesheet: Native style '%s' not available", native_style);
+		}
+	}
 	else
 	{
 		QString stylesheet_path;

rpcs3/rpcs3qt/gui_settings.h

@@ -89,6 +89,7 @@ namespace gui
 	const QString Settings = "CurrentSettings";
 	const QString DefaultStylesheet = "default";
 	const QString NoStylesheet = "none";
+	const QString NativeStylesheet = "native";
 
 	const QString main_window  = "main_window";
 	const QString game_list    = "GameList";

rpcs3/rpcs3qt/settings_dialog.cpp

@@ -11,6 +11,7 @@
 #include <QSpinBox>
 #include <QTimer>
 #include <QScreen>
+#include <QStyleFactory>
 
 #include "gui_settings.h"
 #include "display_sleep_control.h"
@@ -2412,6 +2413,13 @@ void settings_dialog::AddStylesheets()
 	ui->combo_stylesheets->clear();
 
 	ui->combo_stylesheets->addItem(tr("None", "Stylesheets"), gui::NoStylesheet);
+
+	for (const QString& key : QStyleFactory::keys())
+	{
+		// Variant value: "native (<style>)"
+		ui->combo_stylesheets->addItem(tr("Native (%0)", "Stylesheets").arg(key), QString("%0 (%1)").arg(gui::NativeStylesheet, key));
+	}
+
 	ui->combo_stylesheets->addItem(tr("Default (Bright)", "Stylesheets"), gui::DefaultStylesheet);
 
 	for (const QString& entry : m_gui_settings->GetStylesheetEntries())