Safari on Windows has poor plugin scrolling behavior (repainting lag) I expect this is a WebKit issue, which is why I'm filing it here instead of Radar. I've attached a page with a bunch of plugins on it. The movie files won't actually be found in bugzilla, but that's OK. Just the "Quicktime Q" shows the issue. The WMV plugin draws outside its bounds when scrolling, and the Q's take a long time to repaint correctly. Firefox has no trouble scrolling this content smoothly. I have not tested Safari on the Mac (my mac is shut off atm).
Created attachment 20913 [details] test case (scroll to see poor repaint behavior)
<rdar://problem/5904958>
Testing with Safari 5.0.3 shows Safari's behavior to be very similar to Firefox's. I can't test with Chrome as this page seems to crash Chrome. Both Safari and Firefox show smearing in the WMP area; I suspect this is due to a bug in the WMP plugin. Both Safari and Firefox show some shearing (not smearing) of the Qs themselves; I suspect this is due to a bug in the QuickTime plugin. The smoothness of scrolling in Firefox and Safari seems pretty indistinguishable.
Filed chrome bug http://code.google.com/p/chromium/issues/detail?id=65643