[DatePicker, RTL, Regression] RTL DatePicker drop down positioned in the wrong place after changing showWeekNumbers property (regression)

Tested with 8u20 b17.

Steps to reproduce:
1. Set the node orientation of DatePicker to RTL.
2. Click the DatePicker, now the drop down is positioned correctly.
3. Change showWeekNumbers property.
4. Click the DatePicker again, the drop down is not aligned to the right side of DatePicker combo box. But from the second time, the position is correct.

This is reproducible for 8u20 only. It didn't happen to 8, 8u5 and 8u11.