OnLastWindowClose
Triggers a script to run each time you close the last window of a database file.
When the script runs
Before the first window of a file is closed.
Modes in which the trigger can be used
OnLastWindowClose activates after FileMaker Pro exits a mode.
Result
You can use the Exit Script script step within the triggered script to return True or False.
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Originated in
FileMaker Pro 12.0
Notes
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The script will run even if the file itself remains open, for example if it is referenced by another file with open windows. The closing script will be performed each time the last open window of the specified file is closed.
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Related topics 
Setting up script triggers
Setting script triggers for layouts
Setting script triggers for objects