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Modal dialogs are popups that interrupt the user's work and require the user to take action. The window showing the dialog does not allow other interaction meanwhile.

  • Modal dialogs are not great design and should be avoided. If users face them often they will get the habit of just "clicking them away", possibly resulting in destructive actions.
  • In some cases they are however acceptable as a way to prevent serious errors. Typically if there's the potential of data-loss.
  • A modal dialog should not be the only way to prevent data-loss. Users may still just "click away" a confirmation dialog without much thinking.
    For example:
    • Auto-saves and Recover Last Session can be used to prevent data-loss when users accidentally closed Blender without saving, ignoring the confirmation prompt.
    • When a user overrides files in Blender's File Browser, the overridden files are moved to the system trash by default, despite the confirmation prompt (Note: Not actually the case at the time of writing).
  • If there is no such secondary way to recover from errors, a modal dialog must not have a default action (the action that happens when pressing ↵ Enter, typically visualized by a button with a blue background).

Also see:

  • Aza Raskin, Never Use a Warning When you Mean Undo [1]
  • Jakob Nielsen, Confirmation Dialogs Can Prevent User Errors — If Not Overused [2]
  • Jef Raskin, The Humane Interface (ISBN 0-201-37937-6)