https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.control.regionchanged?view=netframework-4.8.1&tree=cete.dynamicpdf.viewer.pdfnavigationpanel.regionchanged
Control.RegionChanged Event (System.Windows.Forms) | Microsoft Learn
Occurs when the value of the Region property changes.
event systemwindows formscontrolmicrosoftlearn