Add public QWindow API for safe area margins

The margins are exposed as a simple QMargins, as this is covers the
primary use-cases sufficiently at this point.

A Q_PROPERTY is not added, as this would conflict with the explicit
Qt Quick API that will also be added.

Task-number: QTBUG-125374
Change-Id: I504c3000473de0b09272aa5a3e58ebf9a41aa942
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/corelib/kernel/qcoreevent.cpp

@@ -230,6 +230,8 @@ Q_TRACE_POINT(qtcore, QEvent_dtor, QEvent *event, QEvent::Type type);
     \value WindowUnblocked                  The window is unblocked after a modal dialog exited.
     \value WinIdChange                      The window system identifier for this native widget has changed.
     \value ZOrderChange                     The widget's z-order has changed. This event is never sent to top level windows.
+    \value [since 6.9] SafeAreaMarginsChange
+                                            The window's safe area margins have changed.
 
     User events should have values between \c User and \c{MaxUser}:
 

src/corelib/kernel/qcoreevent.h

@@ -292,6 +292,8 @@ public:
         ParentWindowAboutToChange = 225,
         ParentWindowChange = 226,
 
+        SafeAreaMarginsChange = 227,
+
         // 512 reserved for Qt Jambi's MetaCall event
         // 513 reserved for Qt Jambi's DeleteOnMainThread event
 

src/gui/kernel/qguiapplication.cpp

@@ -2785,10 +2785,10 @@ void QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(QWindowSystemInt
     if (wse->window.isNull())
         return;
 
-    // Handle by forwarding directly to QWindowPrivate, instead of sending spontaneous
-    // QEvent like most other functions, as there's no QEvent type for the safe area
-    // change, and we don't want to add one until we know that this is a good API.
-    qt_window_private(wse->window)->processSafeAreaMarginsChanged();
+    emit wse->window->safeAreaMarginsChanged(wse->window->safeAreaMargins());
+
+    QEvent event(QEvent::SafeAreaMarginsChange);
+    QGuiApplication::sendSpontaneousEvent(wse->window, &event);
 }
 
 void QGuiApplicationPrivate::processThemeChanged(QWindowSystemInterfacePrivate::ThemeChangeEvent *tce)

src/gui/kernel/qwindow.cpp

@@ -1967,6 +1967,37 @@ void QWindow::setFramePosition(const QPoint &point)
     }
 }
 
+/*!
+    Returns the safe area margins of the window.
+
+    The safe area represents the part of the window where content
+    can be safely placed without risk of being obscured by, or
+    conflicting with, other UI elements, such as system UIs.
+
+    The margins are relative to the internal geometry of the
+    window, i.e QRect(0, 0, width(), height()).
+
+    \code
+    void PaintDeviceWindow::paintEvent(QPaintEvent *)
+    {
+        QPainter painter(this);
+        QRect rect(0, 0, width(), height());
+        painter.fillRect(rect, QGradient::SunnyMorning);
+        painter.fillRect(rect - safeAreaMargins(), QGradient::DustyGrass);
+    }
+    \endcode
+
+    \since 6.9
+    \sa geometry()
+*/
+QMargins QWindow::safeAreaMargins() const
+{
+    Q_D(const QWindow);
+    if (d->platformWindow)
+        return QHighDpi::fromNativePixels(d->platformWindow->safeAreaMargins(), this);
+    return {};
+}
+
 /*!
     \brief set the position of the window on the desktop to \a pt
 
@@ -3255,6 +3286,9 @@ QDebug operator<<(QDebug debug, const QWindow *window)
             const QMargins margins = window->frameMargins();
             if (!margins.isNull())
                 debug << ", margins=" << margins;
+            const QMargins safeAreaMargins = window->safeAreaMargins();
+            if (!safeAreaMargins.isNull())
+                debug << ", safeAreaMargins=" << safeAreaMargins;
             debug << ", devicePixelRatio=" << window->devicePixelRatio();
             if (const QPlatformWindow *platformWindow = window->handle())
                 debug << ", winId=0x" << Qt::hex << platformWindow->winId() << Qt::dec;

src/gui/kernel/qwindow.h

@@ -202,6 +202,8 @@ public:
     QPoint framePosition() const;
     void setFramePosition(const QPoint &point);
 
+    QMargins safeAreaMargins() const;
+
     inline int width() const { return geometry().width(); }
     inline int height() const { return geometry().height(); }
     inline int x() const { return geometry().x(); }
@@ -309,6 +311,8 @@ Q_SIGNALS:
     void maximumWidthChanged(int arg);
     void maximumHeightChanged(int arg);
 
+    void safeAreaMarginsChanged(QMargins arg);
+
     void visibleChanged(bool arg);
     Q_REVISION(2, 1) void visibilityChanged(QWindow::Visibility visibility);
     Q_REVISION(2, 1) void activeChanged();

src/gui/kernel/qwindow_p.h

@@ -92,8 +92,6 @@ public:
                          qxp::function_ref<void()> funcWidthChanged,
                          qxp::function_ref<void()> funcHeightChanged);
 
-    virtual void processSafeAreaMarginsChanged() {}
-
     virtual bool participatesInLastWindowClosed() const;
     virtual bool treatAsVisible() const;
 

src/widgets/kernel/qwidgetwindow.cpp

@@ -106,13 +106,6 @@ public:
 
     QRectF closestAcceptableGeometry(const QRectF &rect) const override;
 
-    void processSafeAreaMarginsChanged() override
-    {
-        Q_Q(QWidgetWindow);
-        if (QWidget *widget = q->widget())
-            QWidgetPrivate::get(widget)->updateContentsRect();
-    }
-
     bool participatesInLastWindowClosed() const override;
     bool treatAsVisible() const override;
 };
@@ -391,6 +384,10 @@ bool QWidgetWindow::event(QEvent *event)
         handleDevicePixelRatioChange();
         break;
 
+    case QEvent::SafeAreaMarginsChange:
+        QWidgetPrivate::get(m_widget)->updateContentsRect();
+        break;
+
     default:
         break;
     }