System Usage
Each application has an event queue that receives events from the operating system (for example key presses and mouse button clicks). If an application takes longer than 2 seconds to process the events in its event queue (regardless of the exact cause) the operating system displays the wait cursor whenever the cursor hovers over that application's windows.
This is meant to indicate that the application is temporarily unresponsive, a state from which the application may recover, however it may also indicate that the application has entered an unrecoverable state. This prevents the user from closing, resizing, or even minimizing the windows of that application. Users can choose to terminate an unresponsive application, by using "Force Quit" under the Apple menu, the keystroke command-option-escape, or the Force Quit command found by control-clicking (or right-clicking) the icon of an unresponsive application in the Dock.
While one application is active, typically other applications remain usable in the meantime.
Read more about this topic: Spinning Pinwheel
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