Weird! I disabled Speed Download, removed the plugin manually, rebooted and stillgot the glitch (both with SD proper on my iMac and SD:light on MBP)..
I ran the uninstaller script on both machines and it fixed them.. So maybe if you have an old, disabled copy of a download manager it can interfere with safari somehow.. Glad to have it fixed anyway, SD has been increasingly pointless as my cable speeds have been bumped, hehe.