Hello,
a super minor regression I have found for all the 5.1 Alphas since the 5.0.13.
The menu bar has now a light blue extension all the width of the main window. In 5.0.13, the color is the same as the title bar.
When the paint.net window is maximized, the icon on the right side are between the solid color title bar and the light blue bar and that makes the icon rather odd.
Here are some screen shots
paint.net 5.0.13
paint.net 5.1 Alpha 8902.3265
These images are cropped from maximized windows because 2560 pixels wide are too large.
Good luck