The Cupertino-based Mac maker initiated the testing process by offering software developers access to Mac OS X 10.5.1 build 9B13, which weighed in at a hefty 267MB, according to people familiar with the process.
Among the fixes already baked into the first test build are corrections to Leopard's application Firewall, Spotlight indexing, iCal syncing, Keychain login and text drawing corruption.
Mac OS X 10.5.1 will also deliver a significant number of bug fixes relating to Leopard's email client, namely corrections to Mail syncing, HTML anchors, Email alarms, To-Do Notes, and smart mailboxs.
In total, build 9B13 includes well over two dozen fixes and code corrections, those familiar with the software say, and addresses an issue present when moving files across partitions -- possibly one and the same with the widely publicized Finder data-loss bug.
Apple has reportedly asked that its developers test the update broadly, paying particular attention to Time Machine, Mail, iCal, Back To My Mac, Bonjour, AirPort, gaming graphics, networking and the Finder.
Mac OS X 10.5.1 is expected to be released within a few weeks' time.