Hi Tomasz,
We have analyzed your reported issue with reference of given sample and found that you have update the modified cell value to your data which is bind with Gantt at “endEdit” event. Due to this kind of update Gantt get refresh, we would like to suggest you that use Gantt data source from Gantt instance. It have the updated value.
We have prepared sample with your requirement. Please find the sample from below link,
Otherwise, please let us know the purpose of updating cell value in the endEdit event. It will helpful for us to serve better.
Regards,
Jesus Arockia Sankaran S