Add attribute to enable font and palette propagation in QSS.

By default when using Qt Style Sheets, a widget does not inherit its
font and palette from its parent widget. With the
Qt::AA_UseStyleSheetPropagationInWidgetStyles application attribute set,
propagation when using Qt Style Sheets behaves like it does with regular
QWidget::setPalette() and QWidget::setFont() calls.

[ChangeLog][QtWidgets] Added the
Qt::AA_UseStyleSheetPropagationInWidgetStyles attribute which enables
font and palette propagation for Qt Style Sheets.

Task-number: QTBUG-37580
Change-Id: I3038c13d61e32625a1a05291c5394eaefd376a68
Reviewed-by: Samuel Nevala <samuel.nevala@intopalo.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

7 files changed, 159 insertions, 35 deletions

diff --git a/src/corelib/global/qnamespace.h b/src/corelib/global/qnamespace.h
index e62b658afc..056e519e56 100644
--- a/src/corelib/global/qnamespace.h
+++ b/src/corelib/global/qnamespace.h
@@ -498,6 +498,7 @@ public:
         AA_ShareOpenGLContexts = 18,
         AA_SetPalette = 19,
         AA_NoHighDpiScaling = 20,
+        AA_UseStyleSheetPropagationInWidgetStyles = 21, // ### Qt 6: remove me
 
         // Add new attributes before this line
         AA_AttributeCount
diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index c7effe0c0c..b7c6500831 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -213,6 +213,13 @@
            so this does not guarantee that QPaintDevice::devicePixelRatio() will
            be equal to 1. This value has been added in Qt 5.6.
 
+    \value AA_UseStyleSheetPropagationInWidgetStyles By default, Qt Style Sheets
+           disable regular QWidget palette and font propagation. When this flag
+           is enabled, font and palette changes propagate as though the user had
+           manually called the corresponding QWidget methods. See
+           \l{The Style Sheet Syntax#Inheritance}{The Style Sheet Syntax - Inheritance}
+           for more details. This value has been added in Qt 5.7.
+
     The following values are obsolete:
 
     \value AA_ImmediateWidgetCreation This attribute is no longer fully
diff --git a/src/widgets/doc/snippets/code/doc_src_stylesheet.cpp b/src/widgets/doc/snippets/code/doc_src_stylesheet.cpp
index a937498fe5..01f4c528a2 100644
--- a/src/widgets/doc/snippets/code/doc_src_stylesheet.cpp
+++ b/src/widgets/doc/snippets/code/doc_src_stylesheet.cpp
@@ -138,3 +138,7 @@ emailEdit->setProperty("mandatoryField", true);
 QSpinBox *ageSpinBox = new QSpinBox(this);
 ageSpinBox->setProperty("mandatoryField", true);
 //! [95]
+
+//! [96]
+qApp->setAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles, true);
+//! [97]
diff --git a/src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc b/src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc
index fc3ac345a8..bea1a6e657 100644
--- a/src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc
+++ b/src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc
@@ -492,9 +492,9 @@
     \section1 Inheritance
 
     In classic CSS, when font and color of an item is not explicitly set,
-    it gets automatically inherited from the parent. When using Qt Style Sheets,
-    a widget does \b{not} automatically inherit its font and color setting
-    from its parent widget.
+    it gets automatically inherited from the parent. By default, when using
+    Qt Style Sheets, a widget does \b{not} automatically inherit its font
+    and color setting from its parent widget.
 
     For example, consider a QPushButton inside a QGroupBox:
 
@@ -507,9 +507,23 @@
 
     \snippet code/doc_src_stylesheet.cpp 25
 
-    In contrast, setting a font and propagate using QWidget::setFont() and
+    In contrast, setting a font and palette using QWidget::setFont() and
     QWidget::setPalette() propagates to child widgets.
 
+    If you would prefer that the font and palette propagate to child widgets,
+    you can set the Qt::AA_UseStyleSheetPropagationInWidgetStyles flag, like
+    this:
+
+    Usage:
+    \snippet code/doc_src_stylesheet.cpp 96
+
+    When the widget-style font and palette propagation is enabled, font and
+    palette changes made through Qt Style Sheets will behave as though the
+    user had manually called the corresponding QWidget::setPalette() and
+    QWidget::setFont() methods on all of the QWidgets targeted by the style
+    sheet.  If this would have caused propagation in C++, it will cause
+    propagation in style sheets and visa versa.
+
     \section1 Widgets Inside C++ Namespaces
 
     The Type Selector can be used to style widgets of a particular type. For
diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 1ebf782edf..66ded4cb5f 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -1954,11 +1954,14 @@ void QWidgetPrivate::propagatePaletteChange()
     }
     int mask = data.pal.resolve() | inheritedPaletteResolveMask;
 
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
     QEvent pc(QEvent::PaletteChange);
     QApplication::sendEvent(q, &pc);
     for (int i = 0; i < children.size(); ++i) {
         QWidget *w = qobject_cast<QWidget*>(children.at(i));
-        if (w && !w->testAttribute(Qt::WA_StyleSheet)
+        if (w && (!w->testAttribute(Qt::WA_StyleSheet) || useStyleSheetPropagationInWidgetStyles)
             && (!w->isWindow() || w->testAttribute(Qt::WA_WindowPropagation))) {
             QWidgetPrivate *wd = w->d_func();
             wd->inheritedPaletteResolveMask = mask;
@@ -4543,15 +4546,19 @@ void QWidget::setPalette(const QPalette &palette)
 QPalette QWidgetPrivate::naturalWidgetPalette(uint inheritedMask) const
 {
     Q_Q(const QWidget);
+
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
     QPalette naturalPalette = QApplication::palette(q);
-    if (!q->testAttribute(Qt::WA_StyleSheet)
+    if ((!q->testAttribute(Qt::WA_StyleSheet) || useStyleSheetPropagationInWidgetStyles)
         && (!q->isWindow() || q->testAttribute(Qt::WA_WindowPropagation)
 #ifndef QT_NO_GRAPHICSVIEW
             || (extra && extra->proxyWidget)
 #endif //QT_NO_GRAPHICSVIEW
             )) {
         if (QWidget *p = q->parentWidget()) {
-            if (!p->testAttribute(Qt::WA_StyleSheet)) {
+            if (!p->testAttribute(Qt::WA_StyleSheet) || useStyleSheetPropagationInWidgetStyles) {
                 if (!naturalPalette.isCopyOf(QApplication::palette())) {
                     QPalette inheritedPalette = p->palette();
                     inheritedPalette.resolve(inheritedMask);
@@ -4687,15 +4694,19 @@ void QWidget::setFont(const QFont &font)
 QFont QWidgetPrivate::naturalWidgetFont(uint inheritedMask) const
 {
     Q_Q(const QWidget);
+
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
     QFont naturalFont = QApplication::font(q);
-    if (!q->testAttribute(Qt::WA_StyleSheet)
+    if ((!q->testAttribute(Qt::WA_StyleSheet) || useStyleSheetPropagationInWidgetStyles)
         && (!q->isWindow() || q->testAttribute(Qt::WA_WindowPropagation)
 #ifndef QT_NO_GRAPHICSVIEW
             || (extra && extra->proxyWidget)
 #endif //QT_NO_GRAPHICSVIEW
             )) {
         if (QWidget *p = q->parentWidget()) {
-            if (!p->testAttribute(Qt::WA_StyleSheet)) {
+            if (!p->testAttribute(Qt::WA_StyleSheet) || useStyleSheetPropagationInWidgetStyles) {
                 if (!naturalFont.isCopyOf(QApplication::font())) {
                     QFont inheritedFont = p->font();
                     inheritedFont.resolve(inheritedMask);
@@ -4747,6 +4758,8 @@ void QWidgetPrivate::updateFont(const QFont &font)
 #ifndef QT_NO_STYLE_STYLESHEET
     const QStyleSheetStyle* cssStyle;
     cssStyle = extra ? qobject_cast<const QStyleSheetStyle*>(extra->style) : 0;
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
 #endif
 
     data.fnt = QFont(font, q);
@@ -4771,7 +4784,7 @@ void QWidgetPrivate::updateFont(const QFont &font)
         if (w) {
             if (0) {
 #ifndef QT_NO_STYLE_STYLESHEET
-            } else if (w->testAttribute(Qt::WA_StyleSheet)) {
+            } else if (!useStyleSheetPropagationInWidgetStyles && w->testAttribute(Qt::WA_StyleSheet)) {
                 // Style sheets follow a different font propagation scheme.
                 if (cssStyle)
                     cssStyle->updateStyleSheetFont(w);
@@ -4786,7 +4799,7 @@ void QWidgetPrivate::updateFont(const QFont &font)
     }
 
 #ifndef QT_NO_STYLE_STYLESHEET
-    if (cssStyle) {
+    if (!useStyleSheetPropagationInWidgetStyles && cssStyle) {
         cssStyle->updateStyleSheetFont(q);
     }
 #endif
@@ -10442,7 +10455,11 @@ void QWidget::setParent(QWidget *parent, Qt::WindowFlags f)
 
     d->reparentFocusWidgets(oldtlw);
     setAttribute(Qt::WA_Resized, resized);
-    if (!testAttribute(Qt::WA_StyleSheet)
+
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
+    if (!useStyleSheetPropagationInWidgetStyles && !testAttribute(Qt::WA_StyleSheet)
         && (!parent || !parent->testAttribute(Qt::WA_StyleSheet))) {
         d->resolveFont();
         d->resolvePalette();
diff --git a/src/widgets/styles/qstylesheetstyle.cpp b/src/widgets/styles/qstylesheetstyle.cpp
index 380b2b3939..c9440efc8c 100644
--- a/src/widgets/styles/qstylesheetstyle.cpp
+++ b/src/widgets/styles/qstylesheetstyle.cpp
@@ -2556,7 +2556,13 @@ void QStyleSheetStyle::setPalette(QWidget *w)
         { PseudoClass_Enabled, QPalette::Inactive }
     };
 
-    QPalette p = w->palette();
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
+    QPalette p;
+    if (!useStyleSheetPropagationInWidgetStyles)
+        p = w->palette();
+
     QWidget *ew = embeddedWidget(w);
 
     for (int i = 0; i < 3; i++) {
@@ -2573,32 +2579,84 @@ void QStyleSheetStyle::setPalette(QWidget *w)
         rule.configurePalette(&p, map[i].group, ew, ew != w);
     }
 
-    styleSheetCaches->customPaletteWidgets.insert(w, w->palette());
-    w->setPalette(p);
-    if (ew != w)
-        ew->setPalette(p);
+    if (!useStyleSheetPropagationInWidgetStyles || p.resolve() != 0) {
+        QPalette wp = w->palette();
+        styleSheetCaches->customPaletteWidgets.insert(w, qMakePair(wp, p.resolve()));
+
+        if (useStyleSheetPropagationInWidgetStyles) {
+            p = p.resolve(wp);
+            p.resolve(p.resolve() | wp.resolve());
+        }
+
+        w->setPalette(p);
+        if (ew != w)
+            ew->setPalette(p);
+    }
 }
 
 void QStyleSheetStyle::unsetPalette(QWidget *w)
 {
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
+
     if (styleSheetCaches->customPaletteWidgets.contains(w)) {
-        QPalette p = styleSheetCaches->customPaletteWidgets.value(w);
-        w->setPalette(p);
+        QPair<QPalette, uint> p = styleSheetCaches->customPaletteWidgets.value(w);
+        styleSheetCaches->customPaletteWidgets.remove(w);
+
+        QPalette original = p.first;
+
+        if (useStyleSheetPropagationInWidgetStyles) {
+            original.resolve(original.resolve() & p.second);
+
+            QPalette wp = w->palette();
+            wp.resolve(wp.resolve() & ~p.second);
+            wp.resolve(original);
+            wp.resolve(wp.resolve() | original.resolve());
+            original = wp;
+        }
+
+        w->setPalette(original);
         QWidget *ew = embeddedWidget(w);
         if (ew != w)
-            ew->setPalette(p);
-        styleSheetCaches->customPaletteWidgets.remove(w);
+            ew->setPalette(original);
     }
-    QVariant oldFont = w->property("_q_styleSheetWidgetFont");
-    if (oldFont.isValid()) {
-        w->setFont(qvariant_cast<QFont>(oldFont));
+
+    if (useStyleSheetPropagationInWidgetStyles) {
+        unsetStyleSheetFont(w);
+        QWidget *ew = embeddedWidget(w);
+        if (ew != w)
+            unsetStyleSheetFont(ew);
+    } else {
+        QVariant oldFont = w->property("_q_styleSheetWidgetFont");
+        if (oldFont.isValid()) {
+            w->setFont(qvariant_cast<QFont>(oldFont));
+        }
     }
+
     if (styleSheetCaches->autoFillDisabledWidgets.contains(w)) {
         embeddedWidget(w)->setAutoFillBackground(true);
         styleSheetCaches->autoFillDisabledWidgets.remove(w);
     }
 }
 
+void QStyleSheetStyle::unsetStyleSheetFont(QWidget *w) const
+{
+    if (styleSheetCaches->customFontWidgets.contains(w)) {
+        QPair<QFont, uint> f = styleSheetCaches->customFontWidgets.value(w);
+        styleSheetCaches->customFontWidgets.remove(w);
+
+        QFont original = f.first;
+        original.resolve(original.resolve() & f.second);
+
+        QFont font = w->font();
+        font.resolve(font.resolve() & ~f.second);
+        font.resolve(original);
+        font.resolve(font.resolve() | original.resolve());
+
+        w->setFont(font);
+    }
+}
+
 static void updateObjects(const QList<const QObject *>& objects)
 {
     if (!styleSheetCaches->styleRulesCache.isEmpty() || !styleSheetCaches->hasStyleRuleCache.isEmpty() || !styleSheetCaches->renderRulesCache.isEmpty()) {
@@ -2658,6 +2716,7 @@ void QStyleSheetStyleCaches::objectDestroyed(QObject *o)
     hasStyleRuleCache.remove(o);
     renderRulesCache.remove(o);
     customPaletteWidgets.remove((const QWidget *)o);
+    customFontWidgets.remove(static_cast<QWidget *>(o));
     styleSheetCache.remove(o);
     autoFillDisabledWidgets.remove((const QWidget *)o);
 }
@@ -5846,24 +5905,42 @@ void QStyleSheetStyle::updateStyleSheetFont(QWidget* w) const
     // we should never override it.
     if (w->objectName() == QLatin1String("qt_fontDialog_sampleEdit"))
         return;
+
     QWidget *container = containerWidget(w);
     QRenderRule rule = renderRule(container, PseudoElement_None,
             PseudoClass_Active | PseudoClass_Enabled | extendedPseudoClass(container));
-    QFont font = rule.font.resolve(w->font());
 
-    if ((!w->isWindow() || w->testAttribute(Qt::WA_WindowPropagation))
-        && isNaturalChild(w) && qobject_cast<QWidget *>(w->parent())) {
+    const bool useStyleSheetPropagationInWidgetStyles =
+        QCoreApplication::testAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
 
-        font = font.resolve(static_cast<QWidget *>(w->parent())->font());
-    }
+    if (useStyleSheetPropagationInWidgetStyles) {
+        unsetStyleSheetFont(w);
 
-    if (w->data->fnt == font)
-        return;
+        if (rule.font.resolve()) {
+            QFont wf = w->font();
+            styleSheetCaches->customFontWidgets.insert(w, qMakePair(wf, rule.font.resolve()));
+
+            QFont font = rule.font.resolve(wf);
+            font.resolve(wf.resolve() | rule.font.resolve());
+            w->setFont(font);
+        }
+    } else {
+        QFont font = rule.font.resolve(w->font());
 
-    w->data->fnt = font;
+        if ((!w->isWindow() || w->testAttribute(Qt::WA_WindowPropagation))
+            && isNaturalChild(w) && qobject_cast<QWidget *>(w->parent())) {
 
-    QEvent e(QEvent::FontChange);
-    QApplication::sendEvent(w, &e);
+            font = font.resolve(static_cast<QWidget *>(w->parent())->font());
+        }
+
+        if (w->data->fnt == font)
+            return;
+
+        w->data->fnt = font;
+
+        QEvent e(QEvent::FontChange);
+        QApplication::sendEvent(w, &e);
+    }
 }
 
 void QStyleSheetStyle::saveWidgetFont(QWidget* w, const QFont& font) const
diff --git a/src/widgets/styles/qstylesheetstyle_p.h b/src/widgets/styles/qstylesheetstyle_p.h
index 9cd8cb889d..f622f7f419 100644
--- a/src/widgets/styles/qstylesheetstyle_p.h
+++ b/src/widgets/styles/qstylesheetstyle_p.h
@@ -149,6 +149,7 @@ private:
     void unsetPalette(QWidget *);
     void setProperties(QWidget *);
     void setGeometry(QWidget *);
+    void unsetStyleSheetFont(QWidget *) const;
     QVector<QCss::StyleRule> styleRules(const QObject *obj) const;
     bool hasStyleRule(const QObject *obj, int part) const;
 
@@ -178,9 +179,12 @@ public:
     QHash<const QObject *, QHash<int, bool> > hasStyleRuleCache;
     typedef QHash<int, QHash<quint64, QRenderRule> > QRenderRules;
     QHash<const QObject *, QRenderRules> renderRulesCache;
-    QHash<const QWidget *, QPalette> customPaletteWidgets; // widgets whose palette we tampered
     QHash<const void *, QCss::StyleSheet> styleSheetCache; // parsed style sheets
     QSet<const QWidget *> autoFillDisabledWidgets;
+    // widgets whose palettes and fonts we have tampered. stored value pair is
+    // QPair<old widget value, resolve mask of stylesheet value>
+    QHash<const QWidget *, QPair<QPalette, uint> > customPaletteWidgets;
+    QHash<const QWidget *, QPair<QFont, uint> > customFontWidgets;
 };