A common use case for our users is to listen to columns resizing to act upon them, many times is important to understand as well the source of the event. ie User resized the column, through the column menu UI
To allow for this, in ag-grid you can listen to: onColumnResized
This event will also provide you with a params.source, which a string that represents the cause for this event.
An example of this can be in the example below (make sure to open the console)