The following three test cases demonstrate inconsistent and random behaviour
when selecting cells that have a ComboBox. I used Solaris 10 and 6u20.
it seems the issue is OS independent though:

Launch the SwingSet2 Demo. Select the Table Demo icon (third from right)

First test case:
- In the first row, select the "Green" cell. A ComboBox will pop up.
- Click again so that the ComboBox disappears. Only the the triangle
  should remain.
- Now select a different cell in the same column. E.g in the third row
  the "Black" cell. The ComboBox will sometimes pop up and stay up.
  Sometimes it will pop up very briefly but mostly it will not pop up
  at all and a second click is needed.
- Expected behaviour: ComboBox should pop up and stay up always.

Second test case

- In the first row, select the "Green" cell. A ComboBox will pop up and stay up.
- Now select a different cell in the same column which is not currently
  hidden by the open ComboBox. E.g in the tenth row the "Violet" cell.
  The ComboBox will sometimes pop up very briefly but mostly it will not pop up
  at all and a second click is needed.
- Expected behaviour: ComboBox should pop up and stay up always.

Third test case

- In the first row hit the "Brazil" cell. The first row becomes selected.
- In the second or any other row, hit one of the favorite color cells, e.g.
  the "Blue" cell in row 8. A ComboBox will pop up and stay up.
- This is the expected behaviour.