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Warning messages and informational messages don't show up in the center but at one of the borders of the central view area. They show up and may be dismissed, but they don't have to - they automatically disappear after a short delay. The difference between warning and informational message is that warning messages show up more colorful to get your attention and also light up a warning system messages icon indicating new messages (see #Dealing with multiple messages below).
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So even for error messages, only the last message will always be visible - all the others are auto-dismissed. In order to avoid that warnings and errors are missed, not dismissed messages cause the warning system messages icon to light up. The color of this icon shall indicate the highest level of message waiting to be confirmed and a badge shall show the total number of errors and warnings which need to be read.
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