Thanks for your description, I understand your requirement.
You means when you pan map(without mouse release), the new blank area should be filled by tile image.
I think our map don't support it, because when the mouse get release, the map will draw again, and we won't render the area out of current extent, it's for get better performance.
If you think the blank looks bad, you can try to set back color like this:
winformsMap1.BackColor = Color.FromArgb(160, 207, 235);
Wish that's helpful.