Add QPlatformTheme::ScrollSingleStepDistance theme hint

The hard coded value of 20 used in various QAbstractScrollArea
subclasses does not represent the default behavior on all of
our platforms. On macOS e.g. the native NSScrollView has a
default line scroll value of 10 pixels.

The default value isn't changed for macOS in this patch, as it
has to be done in coordination with other changes to make the
behavior change atomic.

Task-number: QTBUG-130667
Change-Id: I532dbd3e2e946a00f426475adfa851d35a656c1b
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -158,6 +158,10 @@ QT_BEGIN_NAMESPACE
     \value MouseCursorSize (QSize) Size of the mouse cursor.
            This enum value has been added in Qt 6.5.
 
+    \value ScrollSingleStepDistance (int) The distance in logical pixels that scrollable
+           controls should scroll in response to a single step (e.g. scroll-bar arrow click,
+           mouse wheel line).
+
     \sa themeHint(), QStyle::pixelMetric()
 */
 
@@ -710,6 +714,8 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
         return false;
     case MenuSelectionWraps:
         return true;
+    case ScrollSingleStepDistance:
+        return 20;
     }
 
     return QVariant();

src/gui/kernel/qplatformtheme.h

@@ -100,6 +100,7 @@ public:
         ShowIconsInMenus,
         PreferFileIconFromTheme,
         MenuSelectionWraps,
+        ScrollSingleStepDistance,
     };
     Q_ENUM(ThemeHint)
 

src/widgets/graphicsview/qgraphicsview.cpp

@@ -389,6 +389,8 @@ void QGraphicsViewPrivate::recalculateContentSize()
     const qreal oldLeftIndent = leftIndent;
     const qreal oldTopIndent = topIndent;
 
+    const auto singleStep = defaultSingleStep();
+
     // If the whole scene fits horizontally, we center the scene horizontally,
     // and ignore the horizontal scroll bars.
     const int left =  q_round_bound(viewRect.left());
@@ -413,7 +415,7 @@ void QGraphicsViewPrivate::recalculateContentSize()
 
         hbar->setRange(left, right);
         hbar->setPageStep(width);
-        hbar->setSingleStep(width / 20);
+        hbar->setSingleStep(width / singleStep);
 
         if (oldLeftIndent != 0)
             hbar->setValue(-oldLeftIndent);
@@ -443,7 +445,7 @@ void QGraphicsViewPrivate::recalculateContentSize()
 
         vbar->setRange(top, bottom);
         vbar->setPageStep(height);
-        vbar->setSingleStep(height / 20);
+        vbar->setSingleStep(height / singleStep);
 
         if (oldTopIndent != 0)
             vbar->setValue(-oldTopIndent);

src/widgets/widgets/qabstractscrollarea.cpp

@@ -24,6 +24,9 @@
 #include "qscrollbar_p.h"
 #include <qwidget.h>
 
+#include <private/qguiapplication_p.h>
+#include <qpa/qplatformtheme.h>
+
 #include <private/qapplication_p.h>
 
 #ifdef Q_OS_WIN
@@ -1500,6 +1503,12 @@ void QAbstractScrollArea::setupViewport(QWidget *viewport)
     Q_UNUSED(viewport);
 }
 
+int QAbstractScrollAreaPrivate::defaultSingleStep() const
+{
+    auto *platformTheme = QGuiApplicationPrivate::platformTheme();
+    return platformTheme->themeHint(QPlatformTheme::ScrollSingleStepDistance).value<int>();
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qabstractscrollarea.cpp"

src/widgets/widgets/qabstractscrollarea_p.h

@@ -73,6 +73,8 @@ public:
     inline bool viewportEvent(QEvent *event)
     { return q_func()->viewportEvent(event); }
     QScopedPointer<QObject> viewportFilter;
+
+    int defaultSingleStep() const;
 };
 
 class QAbstractScrollAreaFilter : public QObject

src/widgets/widgets/qmdiarea.cpp

@@ -1170,6 +1170,8 @@ void QMdiAreaPrivate::updateScrollBars()
     const int startX = q->isLeftToRight() ? childrenRect.left() : viewportRect.right()
                                                                   - childrenRect.right();
 
+    const auto singleStep = defaultSingleStep();
+
     // Horizontal scroll bar.
     if (isSubWindowsTiled && hbar->value() != 0)
         hbar->setValue(0);
@@ -1177,7 +1179,7 @@ void QMdiAreaPrivate::updateScrollBars()
     hbar->setRange(qMin(0, xOffset),
                    qMax(0, xOffset + childrenRect.width() - viewportRect.width()));
     hbar->setPageStep(childrenRect.width());
-    hbar->setSingleStep(childrenRect.width() / 20);
+    hbar->setSingleStep(childrenRect.width() / singleStep);
 
     // Vertical scroll bar.
     if (isSubWindowsTiled && vbar->value() != 0)
@@ -1186,7 +1188,7 @@ void QMdiAreaPrivate::updateScrollBars()
     vbar->setRange(qMin(0, yOffset),
                    qMax(0, yOffset + childrenRect.height() - viewportRect.height()));
     vbar->setPageStep(childrenRect.height());
-    vbar->setSingleStep(childrenRect.height() / 20);
+    vbar->setSingleStep(childrenRect.height() / singleStep);
 }
 
 /*!

src/widgets/widgets/qplaintextedit.cpp

@@ -781,7 +781,7 @@ void QPlainTextEditPrivate::init(const QString &txt)
     if (!txt.isEmpty())
         control->setPlainText(txt);
 
-    hbar->setSingleStep(20);
+    hbar->setSingleStep(defaultSingleStep());
     vbar->setSingleStep(1);
 
     viewport->setBackgroundRole(QPalette::Base);

src/widgets/widgets/qscrollarea.cpp

@@ -101,13 +101,8 @@ QT_BEGIN_NAMESPACE
     \sa setWidget()
 */
 QScrollArea::QScrollArea(QWidget *parent)
-    : QAbstractScrollArea(*new QScrollAreaPrivate,parent)
+    : QScrollArea(*new QScrollAreaPrivate, parent)
 {
-    Q_D(QScrollArea);
-    d->viewport->setBackgroundRole(QPalette::NoRole);
-    d->vbar->setSingleStep(20);
-    d->hbar->setSingleStep(20);
-    d->layoutChildren();
 }
 
 /*!
@@ -118,8 +113,9 @@ QScrollArea::QScrollArea(QScrollAreaPrivate &dd, QWidget *parent)
 {
     Q_D(QScrollArea);
     d->viewport->setBackgroundRole(QPalette::NoRole);
-    d->vbar->setSingleStep(20);
-    d->hbar->setSingleStep(20);
+    const auto singleStep = d->defaultSingleStep();
+    d->vbar->setSingleStep(singleStep);
+    d->hbar->setSingleStep(singleStep);
     d->layoutChildren();
 }
 

src/widgets/widgets/qtextedit.cpp

@@ -170,8 +170,9 @@ void QTextEditPrivate::init(const QString &html)
     if (!html.isEmpty())
         control->setHtml(html);
 
-    hbar->setSingleStep(20);
-    vbar->setSingleStep(20);
+    const auto singleStep = defaultSingleStep();
+    hbar->setSingleStep(singleStep);
+    vbar->setSingleStep(singleStep);
 
     viewport->setBackgroundRole(QPalette::Base);
     q->setMouseTracking(true);