Fixed DrawHorizontalShadowTransparent (vertical was already fixed).

Terminal.Gui/View/Adornment/ShadowView.cs

@@ -109,7 +109,7 @@ internal class ShadowView : View
         Rectangle screen = ViewportToScreen (viewport);
 
         // Fill the rest of the rectangle - note we skip the last since vertical will draw it
-        for (int i = screen.X + 1; i < screen.X + screen.Width - 1; i++)
+        for (int i = Math.Max(0, screen.X + 1); i < screen.X + screen.Width - 1; i++)
         {
             Driver.Move (i, screen.Y);