Fix redundant clearing of the windows with non-default bg color

src/tui/tui.go

@@ -412,7 +412,7 @@ type BorderCharacter int
 func MakeBorderStyle(shape BorderShape, unicode bool) BorderStyle {
 	if shape == BorderNone {
 		return BorderStyle{
-			shape:       BorderRounded,
+			shape:       shape,
 			top:         ' ',
 			bottom:      ' ',
 			left:        ' ',