Based on discussion https://wpftoolkit.codeplex.com/discussions/455218.
Bottom corner of MessageBox close button is rounded on mouseOver and not rounded on mouseLeave under Windows 7.
Comments: ** Comment from web user: BoucherS **
Bottom corner of MessageBox close button is rounded on mouseOver and not rounded on mouseLeave under Windows 7.
Comments: ** Comment from web user: BoucherS **
Fixed in v2.1.