Clean up adornment thickness-delta helpers

Fix indentation of the Margin.ThicknessChanged handler body and factor
the duplicated viewport-size formula out of
GetViewportForAdornmentThickness and GetViewportFrameForAdornmentThickness
into a shared GetViewportSizeForThickness helper.

No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: harder

Terminal.Gui/ViewBase/View.Adornments.cs

@@ -23,7 +23,7 @@ private void SetupAdornments ()
         // When any adornment's thickness changes, recompute frames and request layout + redraw.
         Margin.ThicknessChanged += (_, _) =>
                                    {
-                                      previousMarginThickness = HandleAdornmentThicknessChanged (Margin, previousMarginThickness);
+                                       previousMarginThickness = HandleAdornmentThicknessChanged (Margin, previousMarginThickness);
                                    };
 
         Border.ThicknessChanged += (_, _) =>
@@ -370,18 +370,19 @@ private Rectangle GetViewportForAdornmentThickness (IAdornment changedAdornment,
     {
         Thickness thickness = GetAdornmentsThicknessForAdornment (changedAdornment, changedThickness);
 
-        return new Rectangle (_viewportLocation,
-                              new Size (Math.Max (0, Frame.Width - thickness.Horizontal), Math.Max (0, Frame.Height - thickness.Vertical)));
+        return new Rectangle (_viewportLocation, GetViewportSizeForThickness (thickness));
     }
 
     private Rectangle GetViewportFrameForAdornmentThickness (IAdornment changedAdornment, Thickness changedThickness)
     {
         Thickness thickness = GetAdornmentsThicknessForAdornment (changedAdornment, changedThickness);
 
-        return new Rectangle (new Point (thickness.Left, thickness.Top),
-                              new Size (Math.Max (0, Frame.Width - thickness.Horizontal), Math.Max (0, Frame.Height - thickness.Vertical)));
+        return new Rectangle (new Point (thickness.Left, thickness.Top), GetViewportSizeForThickness (thickness));
     }
 
+    private Size GetViewportSizeForThickness (Thickness thickness) =>
+        new (Math.Max (0, Frame.Width - thickness.Horizontal), Math.Max (0, Frame.Height - thickness.Vertical));
+
     private Thickness GetAdornmentsThicknessForAdornment (IAdornment changedAdornment, Thickness changedThickness)
     {
         Thickness thickness = Thickness.Empty;