I noticed something odd when dragging a window by its titlebar earlier: if you drag the proxy icon, sometimes you’ll be able to drag the window under the menubar.
To reproduce it, first make sure your window has a proxy icon (i.e. has been saved to a file somewhere). It doesn’t matter what app you use; it seems to work in any Cocoa app. Xcode and TextEdit definitely work.
Now drag the proxy icon upwards, towards the menubar. The first time you drag, it should display the normal behaviour, i.e. dragging the proxy icon itself out of the titlebar. Drop the proxy icon and then try dragging it upwards again, repeatedly. Eventually you’ll be able to drag the window up underneath the menubar (it’ll pop back down when you release the drag). Weird. Here’s a pic.
Update: Reported; #4342098.