Fixes #4854. Adornment SubView LineCanvas auto-join pipeline fix

Terminal.Gui/ViewBase/View.Drawing.cs

@@ -137,18 +137,18 @@ public void Draw (DrawContext? context = null)
             DoDrawContent (context);
 
             // ------------------------------------
-            // Draw the line canvas
-            // Restore the clip before rendering the line canvas and adornment subviews
-            // because they may draw outside the viewport.
+            // Draw adornment SubViews BEFORE rendering LineCanvas so their lines
+            // (merged via LineCanvas.Merge) participate in auto-join.
+            // Restore the clip because adornment subviews may draw outside the viewport.
             SetClip (originalClip);
             originalClip = AddFrameToClip ();
-            Trace.Draw (this.ToIdentifyingString (), "LineCanvas");
-            DoRenderLineCanvas (context);
+            Trace.Draw (this.ToIdentifyingString (), "AdornmentSubViews");
+            DoDrawAdornmentsSubViews (context);
 
             // ------------------------------------
-            // Re-draw the Border and Padding Adornment SubViews
-            // HACK: This is a hack to ensure that the Border and Padding Adornment SubViews are drawn after the line canvas.
-            DoDrawAdornmentsSubViews (context);
+            // Draw the line canvas (includes merged lines from adornment SubViews)
+            Trace.Draw (this.ToIdentifyingString (), "LineCanvas");
+            DoRenderLineCanvas (context);
 
             // ------------------------------------
             // Advance the diagnostics draw indicator
@@ -208,7 +208,13 @@ private void DoDrawAdornmentsSubViews (DrawContext? context)
                     subview.SetNeedsDraw ();
                 }
 
-                LineCanvas.Exclude (new Region (subview.FrameToScreen ()));
+                // Only Exclude SubViews that don't merge their LC into the parent.
+                // SuperViewRendersLineCanvas SubViews contribute LC lines via Merge, and
+                // excluding them would prevent those merged lines from rendering.
+                if (!subview.SuperViewRendersLineCanvas)
+                {
+                    LineCanvas.Exclude (new Region (subview.FrameToScreen ()));
+                }
             }
 
             Region? saved = borderView.AddFrameToClip ();
@@ -750,6 +756,7 @@ public void DrawSubViews (DrawContext? context = null)
             {
                 continue;
             }
+
             LineCanvas.Merge (view.LineCanvas);
             view.LineCanvas.Clear ();
         }

Terminal.Gui/ViewBase/View.NeedsDraw.cs

@@ -144,8 +144,11 @@ internal void ClearNeedsDraw ()
 
         SubViewNeedsDraw = false;
 
-        // This ensures LineCanvas' get redrawn
-        if (!SuperViewRendersLineCanvas)
+        // This ensures LineCanvas' get redrawn.
+        // AdornmentViews skip this because their LC may hold merged SubView lines
+        // that haven't been consumed by the parent's DoDrawAdornmentsSubViews yet.
+        // Those lines are cleared in DoDrawAdornmentsSubViews after merging into the parent's LC.
+        if (!SuperViewRendersLineCanvas && this is not AdornmentView)
         {
             LineCanvas.Clear ();
         }

Tests/UnitTestsParallelizable/ViewBase/Draw/AdornmentSubViewLineCanvasTests.cs

@@ -0,0 +1,78 @@
+// Copilot
+using Terminal.Gui.Tracing;
+using UnitTests;
+
+namespace ViewBaseTests.Draw;
+
+/// <summary>
+///     Tests that SubViews of adornments with <see cref="View.SuperViewRendersLineCanvas"/> = true
+///     get their border lines auto-joined with the parent View's border lines.
+///
+///     BUG (#4854): <see cref="View.DoDrawAdornmentsSubViews"/> runs AFTER
+///     <see cref="View.DoRenderLineCanvas"/> in the draw pipeline, so merged lines arrive too late.
+/// </summary>
+public class AdornmentSubViewLineCanvasTests (ITestOutputHelper output) : TestDriverBase
+{
+    /// <summary>
+    ///     Simplest repro: A 7×3 View with only a top border line. A SubView in the Border
+    ///     adds a double-line segment via SuperViewRendersLineCanvas. The ═ should appear
+    ///     but doesn't because the merge happens after the LineCanvas was already rendered.
+    /// </summary>
+    [Fact]
+    public void BorderSubView_Lines_Not_Rendered ()
+    {
+        using IDisposable tracing = TestLogging.Verbose (output, TraceCategory.Draw);
+
+        IDriver driver = CreateTestDriver (7, 3);
+        driver.Clip = new Region (driver.Screen);
+
+        View parent = new ()
+        {
+            Id = "parent",
+            Driver = driver,
+            Width = 7,
+            Height = 3,
+            BorderStyle = LineStyle.Single
+        };
+        parent.Border.Thickness = new Thickness (0, 1, 0, 0);
+
+        View sub = new ()
+        {
+            Id = "sub",
+            X = 2,
+            Y = 0,
+            Width = 3,
+            Height = 1,
+            SuperViewRendersLineCanvas = true
+        };
+        parent.Border.GetOrCreateView ().Add (sub);
+
+        parent.BeginInit ();
+        parent.EndInit ();
+        parent.Layout ();
+
+        Rectangle subScreen = sub.FrameToScreen ();
+        output.WriteLine ($"subScreen: {subScreen}");
+
+        sub.LineCanvas.AddLine (
+            new Point (subScreen.X, subScreen.Y),
+            3,
+            Orientation.Horizontal,
+            LineStyle.Double);
+
+        output.WriteLine ($"sub.LC.Bounds before Draw: {sub.LineCanvas.Bounds}");
+
+        parent.Draw ();
+
+        output.WriteLine ($"Driver output:");
+        output.WriteLine (driver.ToString ());
+
+        // Expected: ──═══──    (double-line from SubView merged with parent's single-line)
+        DriverAssert.AssertDriverContentsAre (
+            """
+            ──═══──
+            """,
+            output,
+            driver);
+    }
+}