The Command + . was sort of a general "stop" shortcut on Classic MacOS. Most apps with some sort of cancelable operation used it to stop the operation which often caused the modal to be dismissed.
Since old MacOS had cooperative multitasking if an app started some task that never offered control back to the OS it monopolized the system. Many (but not all) apps used the Command + . shortcut to cancel those sorts of tasks.
Since old MacOS had cooperative multitasking if an app started some task that never offered control back to the OS it monopolized the system. Many (but not all) apps used the Command + . shortcut to cancel those sorts of tasks.