Playing a puzzle at http://member.nikoli.com (pay site) opens a new window with no toolbar with a flash movie embedded in it. Under Tiger this worked fine. Under Leopard the flash movie seems to be processing events really slowly - clicking and dragging is *extremely* choppy and sometimes has problems detecting mouse location correctly (I believe it's processing a click as if it originated at a previous event). Showing the toolbar seems to fix this problem, though I'm not sure that works all the time.
Playing a demo puzzle on the public-facing portion of the website works just fine, but those popup windows have a toolbar by default.
This issue is 100% reproducible at that website. I don't know of any other site that opens a non-toolbar popup window with an interactive flash movie so I can't test elsewhere.
NPAPI and Flash plugin is not supported anymore. This can be marked as "RESOLVED WONTFIX". Thanks!