Code Review For RT-39755: ScrollPane introduces distortions when scrolling up & down

[Chien Yang]

Hi Kevin,

Please review the proposed fix:

JIRA: https://javafx-jira.kenai.com/browse/RT-39755
Webrev: http://cr.openjdk.java.net/~ckyang/RT-39755/webrev.00/

- Chien