MapperProxyStyle: Workaround QTBUG-24279

Chaining proxy styles isn't really supported by Qt. Mapper can be
caught by this if the system uses a proxy style like qt5ct. This
change makes the "top-most" proxy style the direct proxy of all
underlying styles, while keeping the top-down base style chain.
Due to the generated child events, we can no longer try to update
common_style on child events, but this isn't neccessary anyway
unless really setting a new base style.

For a qt5ct+fusion environment, this fixes:
- segmented buttons
- touch mode menu icon sizes
- touch mode spin box hit tests.

Author: dg0yt

src/gui/widgets/mapper_proxystyle.cpp

@@ -27,7 +27,6 @@
 #include <QApplication>
 #include <QBrush>
 #include <QByteArray>
-#include <QChildEvent>
 #include <QColor>
 #include <QCommonStyle> // IWYU pragma: keep
 #include <QCoreApplication>
@@ -134,11 +133,26 @@ void MapperProxyStyle::onSettingsChanged()
 	}
 }
 
+void MapperProxyStyle::fixupProxyChain(QStyle* base_style)
+{
+	// QTBUG-24279 QProxyStyle doesn't support proxies
+	// This function makes this proxy style the direct proxy of all underlying
+	// styles, while keeping the top-down base style chain.
+	if (auto* base_proxy_style = qobject_cast<QProxyStyle*>(base_style))
+	{
+		fixupProxyChain(base_proxy_style->baseStyle());
+	}
+	auto* const old_base = baseStyle();
+	old_base->setParent(nullptr);     // Prevent deletion on setBaseStyle().
+	setBaseStyle(base_style);         // Set base_style's proxy and parent to this.
+	old_base->setParent(this);        // Restore healthy ownership.
+}
 
 void MapperProxyStyle::polish(QApplication* application)
 {
 	common_style = qobject_cast<QCommonStyle*>(baseStyle());
 
+	fixupProxyChain(baseStyle());
 	QProxyStyle::polish(application);
 	QApplication::setPalette(default_palette);
 
@@ -207,13 +221,6 @@ void MapperProxyStyle::unpolish(QApplication* application)
 }
 
 
-void MapperProxyStyle::childEvent(QChildEvent* event)
-{
-	if (event->added() || event->removed())
-		common_style = qobject_cast<QCommonStyle*>(baseStyle());
-}
-
-
 void MapperProxyStyle::drawPrimitive(QStyle::PrimitiveElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const
 {
 	auto overridden_element = element;

src/gui/widgets/mapper_proxystyle.h

@@ -27,14 +27,14 @@
 #include <QObject>
 #include <QPalette>
 #include <QPixmap>
+#include <QPointer>
 #include <QProxyStyle>
 #include <QRect>
 #include <QSize>
 #include <QString>
 #include <QStyle>
 
 class QApplication;
-class QChildEvent;
 class QCommonStyle;
 class QPainter;
 class QStyleHintReturn;
@@ -101,11 +101,6 @@ Q_OBJECT
 	void unpolish(QApplication* application) override;
 
 
-protected:
-	void childEvent(QChildEvent* event) override;
-	
-	
-public:
 	/**
 	 * Draws the given primitive element.
 	 * 
@@ -198,7 +193,9 @@ Q_OBJECT
 
 	void onSettingsChanged();
 
-	QCommonStyle* common_style = nullptr;
+	void fixupProxyChain(QStyle* proxy_style);
+	
+	QPointer<QCommonStyle> common_style;
 	QPalette default_palette;
 	ToolBarMetrics toolbar = {};
 	MenuMetrics menu       = {};