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Maybe modal dialogs are redundant on many occasions,
but I believe they have their use cases.
For example, how would you issue a warning about
leaving a secure connection, on a web browser?
Sometimes, it is intentional to annoy the user,
or otherwise you might be making too many choices
In The Humane Interface, Jef suggests a translucent overlay with the message that is dismissed if the user keeps typing, but can easily be retrieved in some way. This lets the user know that the computer wants to tell them something, but does not force them to break focus.
I recently ran into a problem on a Vista system with what I think is probably a non-modal dialog box. The problem was the dialogs failure to take the focus, it just appeared on the taskbar and wasn't noticed for a minute or so. I only noticed it because the application was taking longer than it should to do its operation.
The dialog was actually a windows warning message that pops up that warns you when you're going to do something major like using an app that needs elevated permissions. These dialogs are supposed to pop up and pause everything and make everything else dark to get your attention. However, several times it failed to do so when the windows tuning app called regedit. I had to click on its listing on the taskbar and never clicked on the tuning app at any time.
In summary, non-modal dialogs can be accidentally missed because of not gaining the focus all the time and that is not good if the information is important or somebody doesn't expect there to be any warnings with an action (such as in my story above, I didn't expect anything because it was my first experience with Vista.) And yes I noticed, after I discovered the dialog for the first time that all later appearances it blinked in the taskbar when it first appeared, but if the person wasn't expecting it or weren't at their computer, it would never have been noticed. Edited 2008-02-09 19:49 UTC