The control does not fill in the entire window

[valuex]

Thanks for developing this nice control!
I am trying to use it for pdf preview.
It works well, just a little display problem makes me feel confused.
As shown below, the contol is designed to full fill the entire window.
While after lauching it, a certain blank area appears on top of the control.
How could I remove this part?

IDE: VS 2022
Dot net: .NET Framework 8.0
Templated: Windows Form (.NET Framework)
64bit