You need to update video position along with form resizes (IVMRWindowlessControl9.SetVideoPosition
and similar). DirectShow.NET samples do it:
\Samples\VMR9\BitmapMixer\MainForm.cs
:
private void MainForm_ResizeMove(object sender, EventArgs e)
{
if (windowlessCtrl != null)
{
int hr = windowlessCtrl.SetVideoPosition(null,
DsRect.FromRectangle(this.ClientRectangle));
}
}