Stretch metadata row details across the full row width

The details content was pinned left and the text blob capped at 800px,
leaving dead space to the right of embedded-source text and the
flags/typed sub-grids. Let all three detail shapes stretch and give the
last sub-grid column the leftover width so the details area fills its
host row.

Assisted-by: Claude:claude-fable-5:Claude Code

ILSpy.Tests/Metadata/MetadataRowDetailsTests.cs

@@ -23,6 +23,7 @@ using System.Threading.Tasks;
 
 using Avalonia.Controls;
 using Avalonia.Headless.NUnit;
+using Avalonia.Layout;
 using Avalonia.VisualTree;
 
 using AwesomeAssertions;
@@ -147,6 +148,29 @@ public class MetadataRowDetailsTests
 			.OnlyContain(e => !tab.IsRowDetailsVisible!(e));
 	}
 
+	[AvaloniaTest]
+	public void Details_Content_Stretches_Across_The_Full_Row_Width()
+	{
+		// The details area spans the host row, and its content fills it: a capped or
+		// left-pinned control would leave dead space to the right of the blob text or
+		// sub-grid columns.
+		var text = MetadataRowDetails.BuildTextBlob("blob text");
+		text.HorizontalAlignment.Should().Be(HorizontalAlignment.Stretch);
+		text.MaxWidth.Should().Be(double.PositiveInfinity, "the text blob must not cap its width");
+
+		var flagsGrid = (DataGrid)MetadataRowDetails.BuildFlagsGrid(new List<BitEntry> { new(true, "<0001> bit") });
+		flagsGrid.HorizontalAlignment.Should().Be(HorizontalAlignment.Stretch);
+		flagsGrid.Columns[^1].Width.UnitType.Should().Be(DataGridLengthUnitType.Star,
+			"the meaning column takes the leftover width");
+
+		var detailsGrid = (DataGrid)MetadataRowDetails.BuildDetailsGrid(
+			new List<BitEntry> { new(true, "<0001> bit") },
+			("Value", nameof(BitEntry.Value)), ("Meaning", nameof(BitEntry.Meaning)));
+		detailsGrid.HorizontalAlignment.Should().Be(HorizontalAlignment.Stretch);
+		detailsGrid.Columns[^1].Width.UnitType.Should().Be(DataGridLengthUnitType.Star,
+			"the last column takes the leftover width");
+	}
+
 	[AvaloniaTest]
 	public async Task Double_Tap_Inside_The_Details_Area_Does_Not_Resolve_To_An_Activatable_Row()
 	{

ILSpy/Metadata/MetadataRowDetails.cs

@@ -24,7 +24,6 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
-using Avalonia.Layout;
 using Avalonia.Media;
 
 using ICSharpCode.ILSpy.ViewModels;
@@ -105,6 +104,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			grid.Columns.Add(new DataGridTextColumn {
 				Binding = new Binding(nameof(BitEntry.Meaning)),
 				IsReadOnly = true,
+				Width = new DataGridLength(1, DataGridLengthUnitType.Star),
 			});
 			return grid;
 		}
@@ -117,9 +117,7 @@ namespace ICSharpCode.ILSpy.Metadata
 				Text = text,
 				IsReadOnly = true,
 				TextWrapping = TextWrapping.Wrap,
-				MaxWidth = 800,
 				MaxHeight = 400,
-				HorizontalAlignment = HorizontalAlignment.Left,
 			};
 		}
 
@@ -143,6 +141,10 @@ namespace ICSharpCode.ILSpy.Metadata
 					IsReadOnly = true,
 				});
 			}
+			// The last column absorbs the leftover width so the sub-grid fills the host row
+			// instead of ending in dead space after its auto-sized columns.
+			if (grid.Columns.Count > 0)
+				grid.Columns[^1].Width = new DataGridLength(1, DataGridLengthUnitType.Star);
 			return grid;
 		}
 
@@ -154,7 +156,6 @@ namespace ICSharpCode.ILSpy.Metadata
 			CanUserReorderColumns = false,
 			CanUserSortColumns = false,
 			SelectionMode = DataGridSelectionMode.Single,
-			HorizontalAlignment = HorizontalAlignment.Left,
 		};
 	}
 }