The Sidebar button toggles a sidebar containing bookmarks, your Reading List and Shared Links, which is a current view of links shared out by people you follow on Twitter and LinkedIn. To the right of the Safari window, the Sharing button includes Add to Reading List, add Bookmark and different ways to share out your session to social media and message contacts.
The Tab View button organizes your open browsing sessions into a grid and consolidates the different tabs by site. It also gives you the browsing history of your other iCloud-enabled devices, which is a nice touch.
Mail and iCloud
There are two things that are really noteworthy regarding Yosemite's email client Mail. First of all, Apple has figured out a way around the problem of sending attachments that are too large to be sent or received. Apple calls the technology MailDrop: if an attachment is too big, instead of riding with the message, it is securely transmitted to an iCloud server. The email then includes a link to the file, which can be downloaded later. As long as you're using an iCloud address, and as long as the file you're trying to send is less than 5GB, this works seamlessly.
The other nifty Mail feature is its support for markups and annotations. When sending an email with any supported file, like an image or PDF, you'll see a drop-down arrow appear when you hover your mouse. If you click the arrow and select Markup, you'll be brought into a session that allows you to quickly add notes, illustrations, text or even signatures.
For many years, iCloud stored and synchronized documents and data across Macs and iOS devices. In Yosemite, iCloud documents can be found in the Finder sidebar, and can be added, deleted, dropped into folders and otherwise modified as you would any other Finder content. This is even a feature Apple will make available for Windows users. In essence, Yosemite brings a functional file system front end to the iCloud file service and syncs that data across platforms.
Yosemite is full of other numerous tweaks and additions, such as the new Day view in Calendar and the App Store replacing the Software Update option under the Apple menu.
Better yet, Yosemite contains a bunch of bug fixes that have been irking me for a long time -- most notably, the act of dragging and dropping documents using spring-loaded folders actually works now. Before, while the spring-loaded event would open a Finder window, the contents wouldn't always load until I backed out and reloaded the folder again; that's been fixed.
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