Mavericks: Window across multiple monitors?

I just updates to OS X Mavericks on my iMac 27" late 2012 with an additional monitor.

In previous versions of OS X it was no problem to have one window visible across both monitors, which is very helpful if you work on big spreadsheets. But since the update to Mavericks this seems to be impossible. If I put a window so that parts of it are on monitor 1 and the rest on monitor 2, then only that part is visible that is located on that monitor where the mouse pointer was when i released it after dragging the window. The part covering parts of the other monitor are not displayed after end of dragging.

Is this just a matter of settings? I really hope so. But I did not find anything in the settings.

Can you help me to make all parts of a window visible in all monitors it touches?

Quoting from John Siracusa’s review of Mavericks (emphasis mine):

If you prefer the old behavior, uncheck the “Displays have separate Spaces” checkbox in the Mission Control preference pane. Doing so will also restore the ability to have windows that span more than one display.

So yes, it’s just a setting. It’s been put in the Mission Control pane, rather than Displays, because the effect of this change is to allow you to have different full screen apps on different displays.

The page of the review that I linked to explains the change in more detail (and specifically discusses the window-across-multiple-displays behaviour that you want).