My last post referenced the toolbar in Cozi’s family calendar for entering appointments in natural language. Interestingly, the toolbar’s little “X” button for cancelling appointment entry is something of a placebo control—it’s not strictly necessary, but nevertheless improves usability:

It turns out that the user can implicitly cancel appointment entry and close the toolbar by clicking anywhere on the page outside the highlighted box. They can also press the ESC key. These mechanisms seem to suffice when the text box is empty; the user figures they can click outside the box to close it, and discovers that does exactly what they want.

However, once the user enters some appointment text, the stakes for a mistake increase. In that state, it’s not at all clear whether clicking outside the box will save the appointment or throw it away. By offering a little “X” close box, the user can see a clear, explicit means of cancelling the appointment, and can be confident that a click on that control will produce the expected result.

Here both the implicit and explicit means of closing the toolbar feel necessary to me. The implicit means isn’t clear enough when it really matters, and the explicit means feels too demanding in the common case where the user is just clicking around to explore the product. A casual click inside the toolbar produces the state shown above, and a click outside cancels that state. So both mechanisms play a useful role.