Hi Mike,
Thanks for contacting Syncfusion support.
At present there is no event is triggered while column resizing in TreeGrid. For this we have already logged a feature request regarding this.
A support incident has been created under your account to track the status of this requirement. Please log on to our support website to check for further updates.
https://www.syncfusion.com/account/login?ReturnUrl=/support/directtrac/incidents
Please let us know if you require further assistance on this.
Regards,
Jone Sherine P S