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be for the user to undo the action. For example, in Mail, it would be inadvisable to implement click-through for the Delete button, which deletes a message without providing feedback first, because its resulting action is harmful. You could, however, provide click-through for the Add to Favorites button in the Save dialog because its resulting action is not harmful and is fairly easy to undo.

Scroll Bars and Scrolling Windows

People use scroll bars to view areas of a document or a list that is larger than can fit in the current window. Only the active window can be scrolled. A window can have a horizontal scroll bar, a vertical scroll bar, both, or neither.

Figure 5-13 The elements of a scroll bar

Scroll bar

The scroller size reflects how much of the document is visible; the smaller the scroller, the less of the content the user can see at that time. The scroller represents the relative location, in the whole document, of the portion that can be seen in the window.

If the entire contents of a document is visible in a window, the scroll bars do not contain scrollers. Scroll bars in inactive windows have an inactive appearance. See Figure 5-10 (page 81).

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The user can use scroll bars by doing the following:

■Dragging the scroller: This method is usually the fastest way to move around a document. In Mac OS X, the window’s contents changes in “real time” as the user drags the scroller.

■Clicking a scroll arrow: This means, “Show me more of the document that’s hidden in this direction.” The scroller moves in the direction of the arrow. Each scroll arrow click moves the content one unit; your application determines what one unit equals. For example, a word processor would move a line of text per click, a spreadsheet could move one row or column. To ensure smooth scrolling effects, specify units of the same size throughout a document.

■Clicking or pressing in the scroll track: Clicking advances the document by a windowful (the default) or to the pointer’s hot spot, depending on the user’s choice in General preferences. A “windowful” is the height or width of the window, minus at least one unit of overlap to maintain the user’s context. This unit of overlap should be the same as one scroll arrow unit (for example, a line of text, a row of icons, or part of a picture). The Page Up and Page Down keys also move the document view by a windowful.

Pressing in the scroll track displays consecutive windowfuls of the document, until the location of the indicator catches up to the location of the pointer (or until the user releases the mouse button).

It’s best not to add controls to the scroll-bar area of a window. If you add more than one control to this area, it’s hard for people to distinguish among controls and click the right one. Acceptable additions to the scroll area include a splitter bar and a status bar that shows, for example, the current page. To ensure that window controls are easy to use and understand, it’s best to place the majority of your features in the menus as commands. If you really want to provide additional access to features, consider creating a utility window such as a palette with buttons. Only frequently accessed features that significantly benefit users’ productivity should be elevated to the primary interface.

Utility windows that coexist with other windows and need to use the least amount of screen space possible may use small scroll bars. If a window uses small scroll bars, all other controls within the window content area should also be the smaller version. For more information, see “Using Small Versions of Controls” (page 160).

86Window Appearance and Behavior

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Make sure you don’t use a scroll bar when you should really use a slider. Use sliders to change settings; use scroll bars only for representing the relative position of the visible portion of a document or list. For information about sliders, see “Slider Controls” (page 137).

Automatic Scrolling

Most of the time, the user should be in control of scrolling. Your application must perform automatic scrolling in these four cases:

■When your application performs an operation that results in making a new selection or moving the insertion point (for example, when the user searches for some text and your application locates it), scroll the document to show the new selection.

■When the user enters information from the keyboard at a location not visible within the window (for example, the insertion point is on one page and the user has navigated to another page), scroll the document automatically to incorporate and display the new information.

Your application determines the distance to scroll. In general, a word processor scrolls vertically by a line of text, a database or spreadsheet scrolls by one field, a graphics application scrolls to display an entire object, if possible.

■When the user moves the pointer past the edge of the window while holding down the mouse button to make an extended selection, scroll the document automatically in the direction the pointer moves.

■When the user selects something, scrolls to a new location, and then tries to perform an operation on the selection, scroll so the selection is showing before your application performs the operation.

Whenever your application scrolls a document automatically, move the document only as much as necessary. That is, if part of a selection is showing after the user performs an operation, don’t scroll at all. If your application can scroll in only one direction to reveal the selection, don’t scroll in both.

When autoscrolling to a selection, try to show the selection in context. When the selection is too large to show in its entirety, it might be a good idea to show some context rather than having the selection fill the window.

Apple Computer, Inc. June 2002