Fix potential out-of-bounds or nullptr access

This ammends change baed8534bc1dac36a9d0ef4240fc14398076a192, which
might have introduced a hard to reproduce segmentation fault when the
screen number is out of bounds, or when the QScreen object doesn't
return a valid pointer for QScreen::handle.

As the issue doesn't reliably reproduce, this is a speculative fix that
adds bounds and nullptr checking.

Change-Id: I0cec0a344e80159ee1723d840f207267a608cef4
Fixes: QTBUG-82807
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/widgets/kernel/qtooltip.cpp b/src/widgets/kernel/qtooltip.cpp
index 1ec3612457..45835a2043 100644
--- a/src/widgets/kernel/qtooltip.cpp
+++ b/src/widgets/kernel/qtooltip.cpp
@@ -402,10 +402,10 @@ void QTipLabel::placeTip(const QPoint &pos, QWidget *w)
 #endif //QT_NO_STYLE_STYLESHEET
 
     QPoint p = pos;
-    int screenNumber = getTipScreen(pos, w);
-    QScreen *screen = QGuiApplication::screens().at(screenNumber);
-    if (screen) {
-        const QPlatformScreen *platformScreen = screen->handle();
+    const QScreen *screen = QGuiApplication::screens().value(getTipScreen(pos, w),
+                                                             QGuiApplication::primaryScreen());
+    // a QScreen's handle *should* never be null, so this is a bit paranoid
+    if (const QPlatformScreen *platformScreen = screen ? screen->handle() : nullptr) {
         const QSize cursorSize = QHighDpi::fromNativePixels(platformScreen->cursor()->size(),
                                                             platformScreen);
         QPoint offset(2, cursorSize.height());