On my system (fully up to date Mac OS 10.6.1) I get occasional 'flashes' when I scroll the page up and down - is this normal?
Yeah, unfortunately. It happens at the AWT level - the peer's background is repainted before the lightweight paint code kicks in. You can disable this behavior in Windows via a system property, but there doesn't appear to be any way to turn it off in OS X. I submitted a bug report to Sun about it a long time ago, asking them to make it configurable via a public API, but you can imagine how far I got with that...
