I've reproduced the problem using the _base classes.

First I added a form with the following properties:
desktop = .t.
height = 742
MDI Form = .t.
width = 900
ShowWindow = 2
ScrollBars = 3

Next I added an image control to the form with the following properties:
height = 1190
width = 898
top = 0
left = 0

I then added a textbox control at the top of the image and another at the bottom of the image.

Finally, I added a command button with the following code in the click event.
thisform.lockscreen = .t.
thisform.refresh
thisform.lockscreen = .f.

The form will scroll fine until the button is clicked.