Update position correctly in wxGTK insert-text signal handler

"position" argument is input/output and must point after the newly inserted
text on return, but we didn't do it when handling the insertion specially.

Closes #17591.

src/gtk/textentry.cpp

@@ -120,7 +120,13 @@ wx_gtk_insert_text_callback(GtkEditable *editable,
     }
 
     if ( handled )
+    {
+        // We must update the position to point after the newly inserted text,
+        // as expected by GTK+.
+        *position = text->GetInsertionPoint();
+
         g_signal_stop_emission_by_name (editable, "insert_text");
+    }
 }
 
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