Usability: When the "Close" icon doesn't.

On the Windows platform, there are several icons that can appear on the buttons at the upper-right corner of an application window that have a well-defined, standardized functionality. Examples include a horizontal line to minimize the window, a single rectangle to maximize the window, and, perhaps most commonly, an "x" to close the window.

In most cases, when then "x" button is clicked for the main window of an application, the expected behavior is that the button click closes that application. However, for some applications, a click on the application window's "x" button, instead of closing the application, instead performs a "minimize to system tray" operation. Applications that display an icon in the Windows system tray (or "notification area") often (but not always) have this behavior. Microsoft ActiveSync is an example of such an application:

In general, since the "x" button cannot be used to close (exit) such applications, users wishing to close the application must employ some other (non-standardized) method, such as a menu pick in the application window, or a menu pick in the right-click menu of the app's system tray icon. (Some system tray apps don't allow themselves to be closed at all short of manually killing their process, but that's a separate issue!)

It would be nice if a new standardized button icon were used in place of the "x" icon for the "minimize to system tray" functionality. I propose using a small horizontal line in the lower-right corner of the button; this would follow the convention of the left-aligned horizontal line that appears at the bottom of the standard "minimize" icon. Here's a quick sketch I put together in mspaint.exe:

When appropriate, application designers could use the new "minimize to system tray" icon alongside a standard "x" icon, allowing users to easily either minimize the app to the tray or exit it, as desired. This would be an improvement over the current state of affairs where designers must choose which of two standard behaviors -- "close" or "minimize to tray" -- to assign to the "x" button of the main application window.