Scenario: a pageframe containing a grid linked to a cursor

After passing from VFP7 to VFP8, the first time I switch to this page, the vertical scrollbar is visible but not the scrollbox, so the scrollbar cannot be used.

Now, if I activate another page and then switch back to the one containing the grid, it works fine.

I tried lockscreen, themes, scrollbar = 0 and then = 2, etc etc...

Is this a known VFP8 bug?
Any idea?