After I rearranged my Monitors in Display properties in System Preferences, everything worked like normal. Then I rearranged them so the secondary monitor was above the primary in the Display properties, and it continued to work for a while. This fix is temporary apparently.
Nice job, Apple. (sarcasm)
It's called a bug, and bugs happen.
It's supposed to be fixed with Mac OS X 10.10.3.
It's called testing, and it's supposed to happen extensively especially when you modify code in a particular sub-system, like video.
On another note, while on the phone with Apple Support, I discovered that in System Preferences, Display setting, Arrange, if you make the upper monitor the primary monitor (by dragging the menu bar to the upper monitor) it works like it's supposed to even after you close/reopen Dreamweaver. Not ideal, but a least closer to the way it was. And now my workflow is affected less.
This problem appears to be fixed in OS X 10.10.3.