Yosemite divided dock? I've seen this divided dock twice in Yosemite, but I can't find any documentation on it, and don't understand what it's supposed to represent, nor which conditions cause it to occur.  When I click on the leftmost item, it takes me to that application, the division disappears, and I cannot reproduce the conditions which made it appear in the first place.  Can anyone explain this behavior please?  And what does the mini monitor icon represent in the upper right hand corner of the (in this case) safari icon represent?
This is the Handoﬀ feature on iOS 8 and OSX 10.10. It appears when you are doing something on one device and lets you finish your task on your computer.

You can read more on Handoﬀ and Continuity here.