Hi Svatopluk,
Thank you for using Syncfusion products.
Currently we don't have the event called ActiveDocumentChanging in DocumentContainer and we have logged this as a feature request. We will implement this feature in any of our upcoming releases. We usually have an interval of at least three months between releases. The feature implementation would also greatly depend on the factors such as product design, code compatibility and complexity.
Also If you interested in getting a patch for this feature, then you may create a Direct-Trac incident under the following link.
https://www.syncfusion.com/account/login?ReturnUrl=%2fsupport%2fdirecttrac%2fincidentsPlease let us know if you have any queries.
Regards,
Sakthi