Editor: Fix completion tooltip width calculation

Take the offset of the geometry into account when calculating the
available with for the tooltip. This can happen if you have multiple
monitors and the tooltip is positioned on the right monitor.

amends 30cdae912f

Fixes: QTCREATORBUG-26053
Change-Id: I4116547279f3c6708f55a1e1cf7d4ceb76028f9a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/texteditor/codeassist/genericproposalwidget.cpp

@@ -143,12 +143,13 @@ public:
     // Workaround QTCREATORBUG-11653
     void calculateMaximumWidth()
     {
-        const int desktopWidth = screen()->availableGeometry().width();
+        const QRect screenGeometry = screen()->availableGeometry();
+        const int xOnScreen = this->pos().x() - screenGeometry.x();
         const QMargins widgetMargins = contentsMargins();
         const QMargins layoutMargins = layout()->contentsMargins();
         const int margins = widgetMargins.left() + widgetMargins.right()
                 + layoutMargins.left() + layoutMargins.right();
-        m_label->setMaximumWidth(desktopWidth - this->pos().x() - margins);
+        m_label->setMaximumWidth(qMax(0, screenGeometry.width() - xOnScreen - margins));
     }
 
 private: