To reproduce:
1) Drop a GridPane
2) Select column 0
3) Change the "preferred width" in the Inspector
-> Selection handles do not reflect the width change : BUG
4) Select root object
5) Select GridPane
-> Selection handles *still* not reflect the width change : BUG (in FX)
6) Resize the GridPane
-> Selection handles now reflect the width change

Step #5 shows that the root cause of the pb is in GridPane :
GridPaneDesignInfo class maintains some cached data which are
not updated when row/column constraints are changed.

However SB 1.0 implemented a workaround (it calls GridPane.requestLayout()
to force GridPaneDesignInfo to recompute). I accidentally removed this workaround
when I introduced the TrackerV5 class.