Add attribute to indicate a widget was the target of a style sheet

Qt already has the widget attribute WA_StyleSheet to which indicates that
a widget was subject to a style sheet, but it doesn't indicate that the
widget was actually affected by the style sheet. For example, an application
style sheet will set the WA_StyleSheet attribute on all widgets, even if it
only targets QPushButtons. The WA_StyleSheetTarget new attribute pairs with
WA_StyleSheet to give this extra information.

[ChangeLog][QtWidgets] Added the Qt::WA_StyleSheetTarget attribute to
indicate that a widget was affected by a style sheet.

Change-Id: I7cca18ddec8fbb69f294ae2ef990672a5f4f1d83
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>

src/corelib/global/qnamespace.h

@@ -486,6 +486,8 @@ public:
 
         WA_ContentsMarginsRespectsSafeArea = 130,
 
+        WA_StyleSheetTarget = 131,
+
         // Add new attributes before this line
         WA_AttributeCount
     };

src/corelib/global/qnamespace.qdoc

@@ -1153,7 +1153,12 @@
     the widget's author.
 
     \value WA_StyleSheet Indicates that the widget is styled using a
-    \l{Qt Style Sheets}{style sheet}.
+    \l{Qt Style Sheets}{style sheet}. WA_StyleSheet is set whenever a widget
+    is subject to a style sheet, even if the style sheet did not affect the
+    widget appearance.
+
+    \value WA_StyleSheetTarget Indicates that the widget appearance was modified
+    by a \l{Qt Style Sheets}{style sheet}. WA_StyleSheet will also be set.
 
     \value WA_TabletTracking Indicates that the widget has tablet
     tracking enabled. See QWidget::tabletTracking.

src/widgets/styles/qstylesheetstyle.cpp

@@ -530,6 +530,8 @@ public:
     const QStyleSheetGeometryData *geometry() const { return geo; }
     const QStyleSheetPositionData *position() const { return p; }
 
+    bool hasModification() const;
+
     bool hasPalette() const { return pal != 0; }
     bool hasBackground() const { return bg != 0 && (!bg->pixmap.isNull() || bg->brush.style() != Qt::NoBrush); }
     bool hasGradientBackground() const { return bg && bg->brush.style() >= Qt::LinearGradientPattern
@@ -1444,6 +1446,21 @@ void QRenderRule::configurePalette(QPalette *p, QPalette::ColorGroup cg, const Q
         p->setBrush(cg, QPalette::AlternateBase, pal->alternateBackground);
 }
 
+bool QRenderRule::hasModification() const
+{
+    return hasPalette() ||
+           hasBackground() ||
+           hasGradientBackground() ||
+           !hasNativeBorder() ||
+           !hasNativeOutline() ||
+           hasBox() ||
+           hasPosition() ||
+           hasGeometry() ||
+           hasImage() ||
+           hasFont ||
+           !styleHints.isEmpty();
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Style rules
 #define OBJECT_PTR(x) (static_cast<QObject *>(x.ptr))
@@ -2823,6 +2840,9 @@ void QStyleSheetStyle::polish(QWidget *w)
 #endif
 
     QRenderRule rule = renderRule(w, PseudoElement_None, PseudoClass_Any);
+
+    w->setAttribute(Qt::WA_StyleSheetTarget, rule.hasModification());
+
     if (rule.hasDrawable() || rule.hasBox()) {
         if (w->metaObject() == &QWidget::staticMetaObject
 #if QT_CONFIG(itemviews)
@@ -2909,6 +2929,7 @@ void QStyleSheetStyle::unpolish(QWidget *w)
     w->setProperty("_q_stylesheet_minh", QVariant());
     w->setProperty("_q_stylesheet_maxw", QVariant());
     w->setProperty("_q_stylesheet_maxh", QVariant());
+    w->setAttribute(Qt::WA_StyleSheetTarget, false);
     w->setAttribute(Qt::WA_StyleSheet, false);
     QObject::disconnect(w, 0, this, 0);
 #if QT_CONFIG(scrollarea)

tests/auto/widgets/styles/qstylesheetstyle/tst_qstylesheetstyle.cpp

@@ -98,6 +98,8 @@ private slots:
     void widgetStyle();
     void appStyle();
     void QTBUG11658_cachecrash();
+    void styleSheetTargetAttribute();
+
 private:
     QColor COLOR(const QWidget& w) {
         w.ensurePolished();
@@ -1999,6 +2001,59 @@ void tst_QStyleSheetStyle::widgetStylePropagation()
     QCOMPARE(COLOR(childLabel), childExpectedColor);
 }
 
+void tst_QStyleSheetStyle::styleSheetTargetAttribute()
+{
+    QGroupBox gb;
+    QLabel lb(&gb);
+    QPushButton pb(&lb);
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), false);
+
+    qApp->setStyleSheet("QPushButton { background-color: blue; }");
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), true);
+
+    qApp->setStyleSheet("QGroupBox { background-color: blue; }");
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), true);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), false);
+
+    qApp->setStyleSheet("QGroupBox * { background-color: blue; }");
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), true);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), true);
+
+    qApp->setStyleSheet("* { background-color: blue; }");
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), true);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), true);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), true);
+
+    qApp->setStyleSheet("QLabel { font-size: 32pt; }");
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), true);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), false);
+
+    qApp->setStyleSheet("");
+
+    gb.ensurePolished(); lb.ensurePolished(); pb.ensurePolished();
+    QCOMPARE(gb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(lb.testAttribute(Qt::WA_StyleSheetTarget), false);
+    QCOMPARE(pb.testAttribute(Qt::WA_StyleSheetTarget), false);
+}
+
 QTEST_MAIN(tst_QStyleSheetStyle)
 #include "tst_qstylesheetstyle.moc"