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Adherence to Fitt's Law
Overview
Fitt's Law: "The time to acquire a target is a function of the distance to and size of the target."
According to Fitt's Law, targets placed along the edges of the screen are inherintly "larger," because it's impossible to scroll beyond the edge of a screen. Therefore, targets should be placed along the edges of the screen (corners are even better, and directly under the mouse is best).
Here's how Fitt's Law applies to the File menu (considering vertical mouse movement only):
Scenario 1: File menu at edge of screen (OS X) — overshooting the target vertically is not possible. Flick the mouse upwards to land on the target
Scenario 2: File menu near edge of screen (XP, maximized window) — since overshooting the target vertically is possible, most likely two mouse movements are required: first a flick to get to the top of the screen, then a second, more precise downward movement to land the mouse on the target
Scenario 3: File menu attached to document's window, floating on screen (XP, unmaximized window) — users cannot rely on a quick upwards mouse flick to approach the target. Dexterity is required in every direction in order to land on the target
Now let's consider horizontal mouse movement in addition to vertical mouse movement for the above three examples. In scenarios 1 and 2, the File menu is always in the same location every time you run the same application. Users can memorize the location of the File menu. This is known as "muscle memory," and makes the proper mouse flicks easier over time. In scenario 3, the File menu can really be anywhere on the screen at any time, thereby eliminating any possible benefit from muscle memory.
Fitt's Law belies the common misconception that proximity to a target (the File menu, in this case) is the most important factor governing how quickly that target is accessed.
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Mac OS X
Virtually all application menus are attached to the top of the screen, rather than to the applications' windows (with the exception of applications where the developer intentionally chose to alter this default configuration). This simultaneously adheres to Fitt's Law while reducing screen clutter.
OS X's Apple menu, Spotlight menu, and Dashboard buttons all honor Fitt's law: clicking your mouse in each corner activates those items. In addition, you can assign actions (such as starting your screen saver or any of Exposé's functions) to any screen corner.
Parts highlighted in red indicate that those interface elements adhere to Fitt's Law:
Note: the dock adheres to Fitt's law with clicks, but not with drag-and-drop - items can be dragged under dock icons without highlighting them. See Dock vs. Taskbar, Advanced for more information.
Windows XP
All application menus reside within their application's window, so even when maximized, an application's menu items (File, Edit, etc.) are not at the edge of the screen. Corners in XP are better used than screen edges: the Start Menu is always placed in a corner, which makes it a very easy target. Also, when a window is maximized, its Close, Minimize, and Restore buttons are located in the upper-right corner (the Close button will in fact be in the absolute corner, while the Minimize and Restore buttons will be at the vertical edge of the window). The window's contextual menu icon will be located at the absolute upper-left corner.
Parts highlighted in red indicate that those interface elements adhere to Fitt's Law:
Note: The upper corners of the screen only adhere to Fitt's law when a window is maximized. The section in the middle of the top edge of the window is XP's Keyboard switcher.
Contextual & Drop-down Menus
Mac OS X
Access contextual menus by control-clicking if you have a one-button mouse (or via the right mouse button if you have a multi-button mouse). Some contextual menus can additionally be accessed by click-and-holding (such as those for dock icons), though this behavior is not consistent throughout the system. The Finder has an "action" button that calls up a contextual menu for the selected object, letting you access contextual menus in the filesystem without control-clicking or right-clicking.
When an OS X user clicks on a drop-down menu, the current selection will be placed directly under the mouse. Thus, OS X users barely have to move the mouse to select the next or previous menu items. If the menu is so tall that it can't fit on the screen in its entirety (or if it is near the bottom of the screen), up and down arrows will appear as necessary, letting you scroll through the menu. For very tall menus, the scrolling speed will adjust depending on how close the arrow you move your mouse. This lets you scroll by one item at a time if you only need move a few places in either direction; if you need to move many places, you can scroll very quickly. You can also begin typing the name of the menu item you want, and the selection will jump to that item (you can even type spaces for menu items consisting of more than one word).
Windows XP
Access contextual menus via the right mouse button, by hitting Shift-F10, or by using the "context key." Most newer keyboards have a context key (usually located next to the Windows key) that lets you conveniently call up a context menu without using the mouse.
When an XP user clicks on a drop-down menu, the entire menu will pop up below the mouse (or above, if there isn't enough room). The current selection will be highlighted, but it will not be directly underneath the mouse. Thus, the user will need to move the mouse to some degree to get to the next or previous menu items; the taller the menu, the more they will need to move. This behavior is softened a bit by the fact that users can type the first letter of the item they want, and the menu will snap to that item.
Conclusion
In general, OS X adheres to Fitt's Law better than Windows XP. Accessing contextual menus on either operating system is a similar experience, though drop-down menus on OS X better adhere to Fitt's Law. OS X also makes more extensive use of both screen corners and screen edges. XP's windows only adhere to Fitt's Law regarding screen corners when they are maximized.
Mac OS X: 8
Windows XP: 7
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