Fix diff lines that were never rendered

src/Verify.Terminal.Tests/Data/Fourth/new

@@ -0,0 +1,3 @@
+line one
+line two
+line three

src/Verify.Terminal.Tests/Data/Fourth/old

@@ -0,0 +1,2 @@
+line one
+line two

src/Verify.Terminal.Tests/Data/Third/new

@@ -0,0 +1,3 @@
+feat: add new feature
+
+[tag1]

src/Verify.Terminal.Tests/Data/Third/old

@@ -0,0 +1,2 @@
+feat: add new feature
+[tag1]

src/Verify.Terminal.Tests/Expectations/Rendering/Render.Output_First.verified.txt

@@ -16,6 +16,6 @@ First.received.txt
     12       │-│············message:·"Unexpected·character·'='",
           12 │+│············severity:·Error,
     13       │-│············severity:·Verbose,
-    14    14 │ │········},
-    15    15 │ │····],
+    14    13 │ │········},
+    15    14 │ │····],
 ─────────────┴─┴────────────────────────────────────────────────────────────────

src/Verify.Terminal.Tests/Expectations/Rendering/Render.Output_Fourth.verified.txt

@@ -0,0 +1,10 @@
+────────────────────────────────────────────────────────────────────────────────
+Fourth.received.txt
+────────────────────────────────────────────────────────────────────────────────
+-old snapshot
++new snapshot
+───────────┬─┬──────────────────────────────────────────────────────────────────
+    1    1 │ │line·one
+    2    2 │ │line·two
+         3 │+│line·three
+───────────┴─┴──────────────────────────────────────────────────────────────────

src/Verify.Terminal.Tests/Expectations/Rendering/Render.Output_Second.verified.txt

@@ -6,7 +6,8 @@ Second.received.txt
 ───────────┬─┬──────────────────────────────────────────────────────────────────
     1    1 │ │+-Greeting----+
          2 │+│|·············|
-    3    3 │ │|·Hello·World·|
+    2    3 │ │|·Hello·World·|
          4 │+│|·············|
-    5    5 │ │+-------------+
+    3    5 │ │+-------------+
+    4      │-│
 ───────────┴─┴──────────────────────────────────────────────────────────────────

src/Verify.Terminal.Tests/Expectations/Rendering/Render.Output_Third.verified.txt

@@ -0,0 +1,11 @@
+────────────────────────────────────────────────────────────────────────────────
+Third.received.txt
+────────────────────────────────────────────────────────────────────────────────
+-old snapshot
++new snapshot
+───────────┬─┬──────────────────────────────────────────────────────────────────
+    1    1 │ │feat:·add·new·feature
+         2 │+│
+    2    3 │ │[tag1]
+    3    4 │ │
+───────────┴─┴──────────────────────────────────────────────────────────────────

src/Verify.Terminal.Tests/SnapshotRendererTests.cs

@@ -7,6 +7,8 @@ public class SnapshotRendererTests
     [Expectation("Render")]
     [InlineData("First")]
     [InlineData("Second")]
+    [InlineData("Third")]
+    [InlineData("Fourth")]
     public Task Should_Render_Correctly(string scenario)
     {
         // Given

src/Verify.Terminal.Tests/Verify.Terminal.Tests.csproj

@@ -12,6 +12,10 @@
     <EmbeddedResource Include="Data\Second\old" />
     <EmbeddedResource Include="Data\First\new" />
     <EmbeddedResource Include="Data\First\old" />
+    <EmbeddedResource Include="Data\Third\new" />
+    <EmbeddedResource Include="Data\Third\old" />
+    <EmbeddedResource Include="Data\Fourth\new" />
+    <EmbeddedResource Include="Data\Fourth\old" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="Shouldly" />
     <PackageReference Include="Spectre.Console.Testing" />

src/Verify.Terminal/SnapshotDiff.cs

@@ -34,8 +34,10 @@ public SnapshotDiff(Snapshot snapshot, List<DiffPiece> old, List<DiffPiece> @new
                 {
                     // Found an unchanged line after something that
                     // had been modified or deleted.
+                    // Stop is exclusive, so it must be allowed to reach New.Count,
+                    // otherwise the last line of the snapshot is never rendered.
                     var rangeStart = Math.Max(0, start.Value - contextLines);
-                    var rangeEnd = Math.Min(index + contextLines, New.Count - 1);
+                    var rangeEnd = Math.Min(index + contextLines, New.Count);
                     ranges.Add((rangeStart, rangeEnd));
 
                     start = null;
@@ -55,6 +57,13 @@ public SnapshotDiff(Snapshot snapshot, List<DiffPiece> old, List<DiffPiece> @new
             index++;
         }
 
+        if (start != null)
+        {
+            // The snapshot ended while still processing a modified or deleted
+            // line, so there is no trailing unchanged line to close the range.
+            ranges.Add((Math.Max(0, start.Value - contextLines), New.Count));
+        }
+
         if (ranges.Count == 0)
         {
             return new List<(int Start, int Stop)>();

src/Verify.Terminal/SnapshotRenderer.cs

@@ -80,23 +80,23 @@ public IRenderable Render(SnapshotDiff diff, int contextLines)
                 if (@new.Type == ChangeType.Modified)
                 {
                     // Modified lines
-                    RenderLine(ctx, old, index, '-', Color.Red, true, false);
-                    RenderLine(ctx, @new, index, '+', Color.Green, false, true);
+                    RenderLine(ctx, old, old.Position, null, '-', Color.Red);
+                    RenderLine(ctx, @new, null, @new.Position, '+', Color.Green);
                 }
                 else if (@new.Type == ChangeType.Inserted)
                 {
                     // Inserted
-                    RenderLine(ctx, @new, index, '+', Color.Green, false, true);
+                    RenderLine(ctx, @new, null, @new.Position, '+', Color.Green);
                 }
                 else if (@new.Type == ChangeType.Imaginary)
                 {
-                    // Modified lines
-                    RenderLine(ctx, old, index, '-', Color.Red, true, false);
+                    // Deleted
+                    RenderLine(ctx, old, old.Position, null, '-', Color.Red);
                 }
                 else
                 {
                     // Unchanged
-                    RenderLine(ctx, @new, index, ' ', Color.Grey, true, true);
+                    RenderLine(ctx, @new, old.Position, @new.Position, ' ', Color.Grey);
                 }
             }
 
@@ -114,14 +114,18 @@ public IRenderable Render(SnapshotDiff diff, int contextLines)
 
     private void RenderLine(
         SnapshotRendererContext ctx, DiffPiece piece,
-        int index, char op, Color color,
-        bool showLeft, bool showRight)
+        int? leftPosition, int? rightPosition, char op, Color color)
     {
-        var leftLineNumber = showLeft ? (index + 1).ToString() : string.Empty;
-        var rightLineNumber = showRight ? (index + 1).ToString() : string.Empty;
+        // The positions come from the snapshots themselves rather than from the
+        // index into the aligned diff, which drifts once a line is added or removed.
+        var leftLineNumber = leftPosition?.ToString() ?? string.Empty;
+        var rightLineNumber = rightPosition?.ToString() ?? string.Empty;
 
         var maxWidth = _console.Profile.Width - (4 + ctx.LineNumberWidth + 4 + ctx.LineNumberWidth + 1 + 1 + 1 + 1);
-        var pieces = (piece.Text.Length / maxWidth) + ((piece.Text.Length % maxWidth) == 0 ? 0 : 1);
+
+        // An empty line still occupies a single row. Without the minimum of one,
+        // added or removed blank lines would not be rendered at all.
+        var pieces = Math.Max(1, (piece.Text.Length / maxWidth) + ((piece.Text.Length % maxWidth) == 0 ? 0 : 1));
 
         for (var i = 0; i < pieces; i++)
         {