Mavericks is famously supposed to improve the multi-display experience:
OS X Mavericks takes full advantage of every display connected to your Mac, giving you even more flexibility to work the way you want. There’s no longer a primary or secondary display — now each has its own menu bar, and the Dock is available on whichever screen you’re working on. You can run a full-screen app on one display and have multiple windows on another display, or run a full‑screen app independently on each display. You can even turn your HDTV into a fully functional second display using AirPlay and Apple TV.
My experience has been that, when not using full-screen, the new design is a regression. It seems to be designed around the idea of using this application on this display and that application on that display. Using one application on multiple displays doesn’t work as well as in previous versions of Mac OS X (or, indeed, classic Mac OS). Apple tried to make a “power user” feature easier to use but ended up making it less powerful and less useful.
Applications no longer remember which display a window was on. Instead, they always seem to open on the display that currently has the menu bar. If I have a document open on my auxiliary display, save it, close it, and double-click its file in the Finder, the window re-opens on the other display.
Likewise, there are some auxiliary windows (Mail and EagleFiler’s Activity windows, various downloads windows) that I like to have off to the side, while working with the main application windows on my main display. Instead, every time I open Mail it puts the Activity window on the main display, right on top of my inbox.
Another problem is that once I move the auxiliary window to the side, where I want it, it becomes a magnet that attracts future primary windows to the wrong display. For example, if an Activity window is on the small display and all the other windows have been closed, the application’s menu bar will be on the small display, which means that documents will open there (and have their windows shrunk) even though I have always specifically placed them on the large display.
These issues also apply when there are no auxiliary windows, for example if you use the same application to work on two different projects. It is natural to want the windows for project A on one display and the windows for project B on the other display. Mavericks wants you to either put all the windows on one display or to group the windows by application rather than by project.
Update (2013-10-30): Pierre Igot reports that you can get the old multi-display behavior back by unchecking Displays have separate spaces in the Mission Control preference pane. I actually saw that option but didn’t think it would have that effect, however it sort of makes sense when you think about it. Unfortunately, this also brings back the Mountain Lion behavior of only being able to use one display when using an app in full screen.
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