Regression: Fix setting of custom cursors for native widgets.

Currently, there is no concept of not having a cursor set on
a Window. Qt::ArrowCursor is always set instead. This causes
bugs when native child widgets are involved, for example setting
a cursor on the native widget's parent no longer works since
the child's Qt::ArrowCursor applies.

Introduce QWindowPrivate::hasCursor tracking whether a cursor
has been explicitly set and clear in QWindow::unsetCursor().

Handle 0 in QPlatformCursor::changeCursor() to mean
"unsetCursor()":
- Windows: Introduce default constructor for QWindowsWindowCursor
  meaning "0". Search for applicable parent cursor in applyCursor.
- XCB: No big changes required, set XCB_CURSOR_NONE for no cursor.
- Other platforms: Assume Qt::ArrowCursor when cursor = 0 is
  passed for now.

Task-number: QTBUG-28879
Change-Id: Id82722592f3cd5fe577a5b64dcc600c85cfea484
Reviewed-by: Jonathan Liu <net147@gmail.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

4 files changed, 53 insertions, 14 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 190d52776a..aa3bb0ed60 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -2629,6 +2629,13 @@ static inline void applyCursor(QWindow *w, QCursor c)
             cursor->changeCursor(&c, w);
 }
 
+static inline void unsetCursor(QWindow *w)
+{
+    if (const QScreen *screen = w->screen())
+        if (QPlatformCursor *cursor = screen->handle()->cursor())
+            cursor->changeCursor(0, w);
+}
+
 static inline void applyCursor(const QList<QWindow *> &l, const QCursor &c)
 {
     for (int i = 0; i < l.size(); ++i) {
@@ -2642,8 +2649,13 @@ static inline void applyWindowCursor(const QList<QWindow *> &l)
 {
     for (int i = 0; i < l.size(); ++i) {
         QWindow *w = l.at(i);
-        if (w->handle() && w->type() != Qt::Desktop)
-            applyCursor(w, w->cursor());
+        if (w->handle() && w->type() != Qt::Desktop) {
+            if (qt_window_private(w)->hasCursor) {
+                applyCursor(w, w->cursor());
+            } else {
+                unsetCursor(w);
+            }
+        }
     }
 }
 
diff --git a/src/gui/kernel/qplatformcursor.cpp b/src/gui/kernel/qplatformcursor.cpp
index 6e2c12bf39..a28617cb37 100644
--- a/src/gui/kernel/qplatformcursor.cpp
+++ b/src/gui/kernel/qplatformcursor.cpp
@@ -95,6 +95,10 @@ QList<QPlatformCursor *> QPlatformCursorPrivate::getInstances()
 
     \a windowCursor is a pointer to the QCursor that should be displayed.
 
+    To unset the cursor of \a window, 0 is passed. This means \a window does not have
+    a cursor set and the cursor of a the first parent window which has a cursor explicitly
+    set or the system default cursor should take effect.
+
     \a window is a pointer to the window currently displayed at QCursor::pos(). Note
     that this may be 0 if the current position is not occupied by a displayed widget.
 
diff --git a/src/gui/kernel/qwindow.cpp b/src/gui/kernel/qwindow.cpp
index 22ad748fb5..b05d6c577e 100644
--- a/src/gui/kernel/qwindow.cpp
+++ b/src/gui/kernel/qwindow.cpp
@@ -284,7 +284,7 @@ void QWindow::setVisible(bool visible)
     }
 
 #ifndef QT_NO_CURSOR
-    if (visible)
+    if (visible && d->hasCursor)
         d->applyCursor();
 #endif
     d->platformWindow->setVisible(visible);
@@ -1947,13 +1947,7 @@ void QWindowPrivate::maybeQuitOnLastWindowClosed()
 void QWindow::setCursor(const QCursor &cursor)
 {
     Q_D(QWindow);
-    d->cursor = cursor;
-    // Only attempt to set cursor and emit signal if there is an actual platform cursor
-    if (d->screen->handle()->cursor()) {
-        d->applyCursor();
-        QEvent event(QEvent::CursorChange);
-        QGuiApplication::sendEvent(this, &event);
-    }
+    d->setCursor(&cursor);
 }
 
 /*!
@@ -1961,7 +1955,8 @@ void QWindow::setCursor(const QCursor &cursor)
  */
 void QWindow::unsetCursor()
 {
-    setCursor(Qt::ArrowCursor);
+    Q_D(QWindow);
+    d->setCursor(0);
 }
 
 /*!
@@ -1975,14 +1970,39 @@ QCursor QWindow::cursor() const
     return d->cursor;
 }
 
+void QWindowPrivate::setCursor(const QCursor *newCursor)
+{
+
+    Q_Q(QWindow);
+    if (newCursor) {
+        const Qt::CursorShape newShape = newCursor->shape();
+        if (newShape <= Qt::LastCursor && hasCursor && newShape == cursor.shape())
+            return; // Unchanged and no bitmap/custom cursor.
+        cursor = *newCursor;
+        hasCursor = true;
+    } else {
+        if (!hasCursor)
+            return;
+        cursor = QCursor(Qt::ArrowCursor);
+        hasCursor = false;
+    }
+    // Only attempt to set cursor and emit signal if there is an actual platform cursor
+    if (screen->handle()->cursor()) {
+        applyCursor();
+        QEvent event(QEvent::CursorChange);
+        QGuiApplication::sendEvent(q, &event);
+    }
+}
+
 void QWindowPrivate::applyCursor()
 {
     Q_Q(QWindow);
     if (platformWindow) {
         if (QPlatformCursor *platformCursor = screen->handle()->cursor()) {
-            QCursor *oc = QGuiApplication::overrideCursor();
-            QCursor c = oc ? *oc : cursor;
-            platformCursor->changeCursor(&c, q);
+            QCursor *c = QGuiApplication::overrideCursor();
+            if (!c && hasCursor)
+                c = &cursor;
+            platformCursor->changeCursor(c, q);
         }
     }
 }
diff --git a/src/gui/kernel/qwindow_p.h b/src/gui/kernel/qwindow_p.h
index 305888d02c..5282c60640 100644
--- a/src/gui/kernel/qwindow_p.h
+++ b/src/gui/kernel/qwindow_p.h
@@ -98,6 +98,7 @@ public:
         , screen(0)
 #ifndef QT_NO_CURSOR
         , cursor(Qt::ArrowCursor)
+        , hasCursor(false)
 #endif
     {
         isWindow = true;
@@ -109,6 +110,7 @@ public:
 
     void maybeQuitOnLastWindowClosed();
 #ifndef QT_NO_CURSOR
+    void setCursor(const QCursor *c = 0);
     void applyCursor();
 #endif
 
@@ -151,6 +153,7 @@ public:
 
 #ifndef QT_NO_CURSOR
     QCursor cursor;
+    bool hasCursor;
 #endif
 };