Scroll event detection (pdfnet)

Scenario: Customers have the option to annotate the PDF page. This is handled as a richtextbox object. However, there is a bug where the user makes an annotation and wants to scroll down manually (drag the scroll down) so that the annotation moves with it. I want to implement a code that detects a scroll event so that the annotation can be drawn and placed correctly before the program scrolls down / up.

What a simple procedure should be, ended not so easy, but I'm used to this evolution in programming ... We can see the WM_VSCROLL message with the spy ++, but we can't find it using the Console.Writeline in wmdproc, which indicates that it is being handled by an event, but for the life of us, we cannot pinpoint which event.

We tried overriding our mistery scroll event both in MainForm and as its parent, but with no success. Also tried to override it in PDFViewCtrl but it prevents us from overriding it. So we're not okay here. We know that the event is being processed, we just can't find where. We use Pdftron and DevExpress, but it's worth noting that we are not using their DE PDFViewer. Our PDFViewCtrl is uploaded to DevExpress Xtraform.