From eba3d84c88a5e856f4e1da61700f2af30be2f407 Mon Sep 17 00:00:00 2001
From: Tom Ransdell <tom.ransdell@avid.com>
Date: Sun, 12 Jul 2020 22:16:33 -0400
Subject: [PATCH] Fix proxy style to support a proxy as a base style

Add code to update any base styles to always use QStyle::setProxy(this).
This allows composing new styles by chaining multiple style proxies together to build more complex styles.
Previously the baseStyle had to be a normal QStyle class, usually one of the default platform styles: "windows", "macintosh", "fusion", etc.

Change-Id: Ie7c2f66355d9b1fe3fe1f0904418f136229742e8
---
 src/widgets/styles/qproxystyle.cpp | 19 +++++++++++++++++++
 src/widgets/styles/qproxystyle.h   |  1 +
 2 files changed, 20 insertions(+)

diff --git a/src/widgets/styles/qproxystyle.cpp b/src/widgets/styles/qproxystyle.cpp
index 5739678932..d8157711da 100644
--- a/src/widgets/styles/qproxystyle.cpp
+++ b/src/widgets/styles/qproxystyle.cpp
@@ -105,6 +105,23 @@ void QProxyStylePrivate::ensureBaseStyle() const
     baseStyle->setParent(const_cast<QProxyStyle*>(q)); // Take ownership
 }
 
+/* \internal
+
+    This function updates the base style's proxy if the given style is also
+    a QProxyStyle.
+    
+    This function ensures each baseStyle::style() always returns the last proxy
+    so one can compose styles by chaining two or more proxy styles together
+    instead of requiring all the new style logic to be in a single proxy style.
+*/
+void QProxyStyle::updateAnyBaseStyleProxies(QStyle *style)
+{
+    while (QProxyStyle *proxy = qobject_cast<QProxyStyle *>(style)) {
+        style = proxy->baseStyle();
+        style->setProxy(this);
+    }
+}
+
 /*!
   Constructs a QProxyStyle object for overriding behavior in the
   specified \a style, or in the default native \l{QApplication::style()}
@@ -120,6 +137,7 @@ QProxyStyle::QProxyStyle(QStyle *style) :
         d->baseStyle = style;
         style->setProxy(this);
         style->setParent(this); // Take ownership
+        updateAnyBaseStyleProxies(style); 
     }
 }
 
@@ -177,6 +195,7 @@ void QProxyStyle::setBaseStyle(QStyle *style)
     if (d->baseStyle) {
         d->baseStyle->setProxy(this);
         d->baseStyle->setParent(this);
+        updateAnyBaseStyleProxies(d->baseStyle); 
     }
 }
 
diff --git a/src/widgets/styles/qproxystyle.h b/src/widgets/styles/qproxystyle.h
index c26d9abf59..0cffe7f703 100644
--- a/src/widgets/styles/qproxystyle.h
+++ b/src/widgets/styles/qproxystyle.h
@@ -99,6 +99,7 @@ protected:
 private:
     Q_DISABLE_COPY(QProxyStyle)
     Q_DECLARE_PRIVATE(QProxyStyle)
+    void updateAnyBaseStyleProxies(QStyle *style);
 };
 
 #endif // QT_NO_STYLE_PROXY
-- 
2.23.0