two-display configuration is required.

Please run the following test example (you don't need HiDPI display mode to reproduce that):
http://cr.openjdk.java.net/~anazarov/8142861/webrev.02/
($JDK9/bin/java -jar jtreg.jar -jdk:$JDK9 -verbose:all -m .)

press "start" => "parent" + "child" pair will appear on the display "1".
Please try to move "child" dialog (fully) to display "2" - you'll see a translucent "child" image which become solid when the mouse button is released (the expected behavior).

Then drag "child" back to "1" and marshal the windows in line (left to right: "parent", "child", with an interval between them). Please drag the "parent" window to "2", you'll see a translucent "child" on "2" when the cursor crosses the border of "1" (see fig.jpg). Drag the cursor a bit back to "1": the translucent child is still visible on "2", and the parent is visible on "1". Release mouse button => the translucent child vanishes instead of appearing on "2" (NOK). Click on parent => child appears on the same display "1" (or not, sometimes).

Checked on Mac OS X 10.10 with JDK9 b91