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Modal dialogs are part of a task flow, and recommendations are given to place them where the focus is in that flow. For example, the window could be placed near the graphical control element that triggers its activation
Using a semi-transparent dark background can obscure information in the main window, so it is best used only when that information would be distracting. A semi-transparent background can be made less intrusive by having the whole background area function as a close button: this is standard on most mobile operating systems, avoids making the user feel trapped, and makes modal windows feel less like malicious pop-ups
Design should follow common practices in the platform the program is running on. Microsoft Windows uses standard controls for modal window dialogs, with affirmative action buttons at the lower right of the panel. Mac OS X uses modal sheets with affirmative action buttons being the right-most command