Avoid focus loss event when wxComboBox popup is opened in wxQt

Prevent wxComboBox from generating a wxEVT_KILL_FOCUS event when the
user opens the drop-down list under wxQt, as logically the drop-down is
part of wxComboBox, even if it's a separate window at Qt level.

Closes https://github.com/wxWidgets/wxWidgets/pull/1377

include/wx/qt/combobox.h

@@ -80,6 +80,7 @@ public:
     virtual void Popup();
     virtual void Dismiss();
 
+    virtual bool QtHandleFocusEvent(QWidget *handler, QFocusEvent *event) wxOVERRIDE;
 protected:
 
     // From wxTextEntry:

src/qt/combobox.cpp

@@ -15,6 +15,9 @@
 
 #include <QtWidgets/QComboBox>
 #include <QtWidgets/QLineEdit>
+#include <QtWidgets/QApplication>
+#include <QtWidgets/QAbstractItemView>
+#include <QtGui/QFocusEvent>
 
 class wxQtComboBox : public wxQtEventSignalHandler< QComboBox, wxComboBox >
 {
@@ -254,6 +257,22 @@ void wxComboBox::Dismiss()
     static_cast<QComboBox *>(GetHandle())->hidePopup();
 }
 
+bool wxComboBox::QtHandleFocusEvent(QWidget *handler, QFocusEvent *event)
+{
+    if ( !event->gotFocus() )
+    {
+        // Qt treats the combobox and its drop-down as distinct widgets, but in
+        // wxWidgets they're both part of the same control, so we have to avoid
+        // generating a lose focus event if the combobox or its drop-down still
+        // have focus.
+        QWidget* const widget = qApp->focusWidget();
+        if ( widget == m_qtComboBox || widget == m_qtComboBox->view() )
+            return false;
+    }
+
+    return wxChoice::QtHandleFocusEvent(handler, event);
+}
+
 void wxComboBox::Clear()
 {
     if ( !IsReadOnly() )