Highlighting of items inside a wxStaticBoxSizer is currently wrong, the item highlights are drawn at wrong positions.
You can produce this by adding a top level element, e.g. a frame, add a vertical wxBoxSizer, add 3 buttons, add a vertical wxStaticBoxSizer and add 3 buttons inside it. Now select a button inside the wxStaticBoxSizer and you see that the red highlight is drawn under one of the buttons outside of the wxStaticBoxSizer.
The change of r2159 uses the wxStaticBox as parent for the items inside a wxStaticBoxSizer, the highlighting code searches the parent element of the item to highlight but uses the wxFormBuilder Object hierarchy and the wxStaticBox isn't part of it. The code will find the panel below the wxStaticBox and draw on it using the coordinates of the item which are relative to its wxWidgets parent which is the wxStaticBox. So if the wxStaticBox isn't the top left most item in the underlying panel the coordinates are wrong, like in the example above.
I tried to fix the wxStaticBoxSizer item rendering problem before you commited your fix and my solution was to convert the item coordinates to screen coordinates and these back to client coordinates of the item where the highlight is drawn on, the attached patch contains this change.
However this doesn't fix the problem i still have here on windows, the highlights are getting overwritten. When running a debug compile of wxFormBuilder you can see that the designer window gets painted multiple times and the highlights get overwritten during some of these paints. I currently don't know if its a problem of wxFormBuilder or wxWidgets, you can however see that the highlights survive in one case: select an item inside a wxStaticBoxSizer, minimize the wxFormBuilder window and restore it, the highlights are there. Select any other item and they disappear again :/.