I have a form that my users fill out. I want the OK button greyed out
until they fill out the form perfectly. At this point, I have
simplified it down to the following:

Private Sub UserForm_Activate()
btnOK.Enabled = False
End Sub

Private Sub txtEnd_AfterUpdate()
If Not IsNull(txtStart) And Not IsNull(txtEnd) Then
btnOK.Enabled = True
End If
End Sub

The problem is the OK button disables fine, but it never enables. When
I step through the code, it enables. Its like the code doesn't have
enough time to "darken in" the OK button, so it skips it unless the
slow human is doing the processing.

What I am doing wrong?