Invalidate selection after deleting wxListBox item with GTK+ 3

For consistency with the other ports, invalidate the selection when
deleting the selected item or any item before it.

Closes #18267.

docs/changes.txt

@@ -113,6 +113,10 @@ All (GUI):
 - Fix wxInfoBar close button size in high DPI (Stefan Ziegler).
 - Make disabling the window before creating it actually work.
 
+wxGTK:
+
+- Invalidate selection after deleting wxListBox item with GTK+ 3 too.
+
 
 3.1.2: (released 2018-12-10)
 ----------------------------

src/gtk/listbox.cpp

@@ -497,6 +497,25 @@ void wxListBox::DoDeleteOneItem(unsigned int n)
     // this returns false if iter is invalid (e.g. deleting item at end) but
     // since we don't use iter, we ignore the return value
     gtk_list_store_remove(m_liststore, &iter);
+
+#ifdef __WXGTK3__
+    // Invalidate selection in a single-selection control for consistency with
+    // MSW and GTK+ 2 where this happens automatically when deleting the
+    // selected item or any item before it.
+    if ( !HasMultipleSelection() )
+    {
+        const int sel = GetSelection();
+        if ( sel != wxNOT_FOUND && static_cast<unsigned>(sel) >= n )
+        {
+            // Don't call SetSelection() from here, it's not totally clear if
+            // it is safe to do, so just do this at GTK+ level.
+            gtk_tree_selection_unselect_all
+            (
+                gtk_tree_view_get_selection(m_treeview)
+            );
+        }
+    }
+#endif // __WXGTK3__
 }
 
 // ----------------------------------------------------------------------------