QPlatformCursor: Add functions for setting/clearing override cursors

QPA is modeled on the assumption that the cursor is a property
of the window and therefore sets the override cursors on all windows.
However, on macOS and Windows, the cursor is set per application (or
screen). On these platforms, the per window cursor setting needs
to be emulated which is a source of bugs especially for override
cursors.

Add new virtuals to QPlatformCursor allowing to set override
cursors which can be implemented by directly setting the cursor
on those platforms.

Task-number: QTBUG-40122
Task-number: QTBUG-61133
Change-Id: I31d6a927128d22bb1620a8ace35988c0e126236e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

3 files changed, 85 insertions, 4 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 7e62ebf161..b76ff65aff 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -3541,6 +3541,22 @@ static inline void applyCursor(const QList<QWindow *> &l, const QCursor &c)
     }
 }
 
+static inline void applyOverrideCursor(const QList<QScreen *> &screens, const QCursor &c)
+{
+    for (QScreen *screen : screens) {
+        if (QPlatformCursor *cursor = screen->handle()->cursor())
+            cursor->setOverrideCursor(c);
+    }
+}
+
+static inline void clearOverrideCursor(const QList<QScreen *> &screens)
+{
+    for (QScreen *screen : screens) {
+        if (QPlatformCursor *cursor = screen->handle()->cursor())
+            cursor->clearOverrideCursor();
+    }
+}
+
 static inline void applyWindowCursor(const QList<QWindow *> &l)
 {
     for (int i = 0; i < l.size(); ++i) {
@@ -3585,7 +3601,10 @@ void QGuiApplication::setOverrideCursor(const QCursor &cursor)
 {
     CHECK_QAPP_INSTANCE()
     qGuiApp->d_func()->cursor_list.prepend(cursor);
-    applyCursor(QGuiApplicationPrivate::window_list, cursor);
+    if (QPlatformCursor::capabilities().testFlag(QPlatformCursor::OverrideCursor))
+        applyOverrideCursor(QGuiApplicationPrivate::screen_list, cursor);
+    else
+        applyCursor(QGuiApplicationPrivate::window_list, cursor);
 }
 
 /*!
@@ -3607,9 +3626,15 @@ void QGuiApplication::restoreOverrideCursor()
     qGuiApp->d_func()->cursor_list.removeFirst();
     if (qGuiApp->d_func()->cursor_list.size() > 0) {
         QCursor c(qGuiApp->d_func()->cursor_list.value(0));
-        applyCursor(QGuiApplicationPrivate::window_list, c);
+        if (QPlatformCursor::capabilities().testFlag(QPlatformCursor::OverrideCursor))
+            applyOverrideCursor(QGuiApplicationPrivate::screen_list, c);
+        else
+            applyCursor(QGuiApplicationPrivate::window_list, c);
     } else {
-        applyWindowCursor(QGuiApplicationPrivate::window_list);
+        if (QPlatformCursor::capabilities().testFlag(QPlatformCursor::OverrideCursor))
+            clearOverrideCursor(QGuiApplicationPrivate::screen_list);
+        else
+            applyWindowCursor(QGuiApplicationPrivate::window_list);
     }
 }
 #endif// QT_NO_CURSOR
diff --git a/src/gui/kernel/qplatformcursor.cpp b/src/gui/kernel/qplatformcursor.cpp
index af0214e016..9d2d65246e 100644
--- a/src/gui/kernel/qplatformcursor.cpp
+++ b/src/gui/kernel/qplatformcursor.cpp
@@ -95,6 +95,17 @@ QT_BEGIN_NAMESPACE
 */
 
 /*!
+    \enum QPlatformCursor::OverrideCursor
+    \since 5.10
+
+    \value OverrideCursor Indicates that the platform implements
+                          QPlatformCursor::setOverrideCursor() and
+                          QPlatformCursor::clearOverrideCursor().
+*/
+
+QPlatformCursor::Capabilities QPlatformCursor::m_capabilities = 0;
+
+/*!
     \fn QPlatformCursor::QPlatformCursor()
 
     Constructs a QPlatformCursor.
@@ -659,4 +670,34 @@ void QPlatformCursorImage::set(const uchar *data, const uchar *mask,
     \brief Return the cursor's hotspot
 */
 
+#ifndef QT_NO_CURSOR
+/*!
+    Reimplement this function in subclass to set an override cursor
+    on the associated screen and return true to indicate success.
+
+    This function can be implemented on platforms where the cursor is a
+    property of the application or the screen rather than a property
+    of the window. On these platforms, the OverrideCursor capability
+    should be set.
+
+    \sa QGuiApplication::setOverrideCursor(), Capabilities
+
+    \since 5.10
+*/
+void QPlatformCursor::setOverrideCursor(const QCursor &)
+{
+}
+
+/*!
+    Reimplement this function in subclass to clear the override cursor.
+
+    \sa QGuiApplication::clearOverrideCursor(), Capabilities
+
+    \since 5.10
+*/
+void QPlatformCursor::clearOverrideCursor()
+{
+}
+#endif // QT_NO_CURSOR
+
 QT_END_NAMESPACE
diff --git a/src/gui/kernel/qplatformcursor.h b/src/gui/kernel/qplatformcursor.h
index dddd9e5831..40e8a562f8 100644
--- a/src/gui/kernel/qplatformcursor.h
+++ b/src/gui/kernel/qplatformcursor.h
@@ -78,21 +78,36 @@ private:
 
 class Q_GUI_EXPORT QPlatformCursor : public QObject {
 public:
+    enum Capability {
+        OverrideCursor = 0x1
+    };
+    Q_DECLARE_FLAGS(Capabilities, Capability)
+
     QPlatformCursor();
 
     // input methods
     virtual void pointerEvent(const QMouseEvent & event) { Q_UNUSED(event); }
 #ifndef QT_NO_CURSOR
     virtual void changeCursor(QCursor * windowCursor, QWindow * window) = 0;
-#endif
+    virtual void setOverrideCursor(const QCursor &);
+    virtual void clearOverrideCursor();
+#endif // QT_NO_CURSOR
     virtual QPoint pos() const;
     virtual void setPos(const QPoint &pos);
 
+    static Capabilities capabilities() { return m_capabilities; }
+    static void setCapabilities(Capabilities c) { m_capabilities = c; }
+    static void setCapability(Capability c) { m_capabilities.setFlag(c); }
+
 private:
     friend void qt_qpa_set_cursor(QWidget * w, bool force);
     friend class QApplicationPrivate;
+
+    static Capabilities m_capabilities;
 };
 
+Q_DECLARE_OPERATORS_FOR_FLAGS(QPlatformCursor::Capabilities)
+
 QT_END_NAMESPACE
 
 #endif // QPLATFORMCURSOR_H