DiffViewer: show only changed lines in revert-hunk preview

The "Revert hunk?" confirmation dialog built its 3-line preview by
taking the first 3 lines of the hunk - which, because DiffService
prepends 3 lines of unchanged context to every hunk, almost always
meant the user saw three lines that weren't even being reverted, and
nothing about the actual change.

Filter the preview to inserted/deleted/modified lines only and prefix
each with the standard diff marker (+/-/~) so the user can see what
will actually be discarded. The overflow indicator (...) now counts
overflow among changed lines rather than total lines, matching what
the user is now reading.

Adds a regression test asserting the preview contains the changed
lines with markers and not the leading context line.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: geevensingh

DiffViewer.Tests/ViewModels/MainViewModelContextMenuTests.cs

@@ -336,6 +336,42 @@ await RunOnUiSyncContextAsync(async vm =>
         settings.Current.SuppressRevertHunkConfirmation.Should().BeTrue();
     }
 
+    [Fact]
+    public async Task RevertHunkAtCaret_PreviewShowsOnlyChangedLinesWithMarkers()
+    {
+        // Repro of the "useless preview" bug: leading context lines were
+        // taking up the entire 3-line preview window, so the user saw three
+        // unchanged lines and zero indication of what was about to be
+        // discarded. The fix filters out Context lines and prefixes diff
+        // markers so the preview actually previews the revert.
+        var repo = new FakeRepositoryService(_repoRoot)
+        {
+            LeftText = "alpha\nbeta\n",
+            RightText = "alpha\ngamma\n",
+        };
+        var git = new FakeGitWriteService();
+        ConfirmationRequest? seen = null;
+
+        await RunOnUiSyncContextAsync(async vm =>
+        {
+            vm.ConfirmHandler = req =>
+            {
+                seen = req;
+                return ConfirmationResult.Cancel();
+            };
+            await vm.DiffPane.LoadAsync(ModifiedRow("a.cs"));
+            var hunk = vm.DiffPane.CurrentHunks.First();
+            vm.RevertHunkAtCaretCommand.Execute(
+                new HunkActionContext(ChangeSide.Right, hunk.NewStartLine));
+        }, repo, git);
+
+        seen.Should().NotBeNull();
+        seen!.Message.Should().Contain("- beta", "deleted line should appear in preview with '-' marker");
+        seen.Message.Should().Contain("+ gamma", "inserted line should appear in preview with '+' marker");
+        seen.Message.Should().NotContain("Preview:\nalpha",
+            "leading context line should be filtered out of preview");
+    }
+
     [Fact]
     public async Task RevertHunkAtCaret_WhenSuppressionOn_SkipsPrompt()
     {

DiffViewer/ViewModels/MainViewModel.cs

@@ -213,12 +213,33 @@ private async Task RevertHunkAtCaret(HunkActionContext? ctx)
         var suppress = _settingsService?.Current.SuppressRevertHunkConfirmation ?? false;
         if (!suppress)
         {
-            var preview = inputs.Hunk.Lines.Take(3).Select(l => l.Text).ToArray();
+            // Preview the lines that will actually be reverted: filter to
+            // inserted/deleted/modified (skip context, which is unchanged
+            // and tells the user nothing about what's being discarded) and
+            // prefix each with the standard diff marker so + / - reads
+            // naturally. The earlier "first 3 lines of the hunk" approach
+            // surfaced only the leading context lines that DiffService adds
+            // around every hunk, so the dialog showed three unchanged
+            // lines and no actual changes - useless for confirming intent.
+            var changedLines = inputs.Hunk.Lines
+                .Where(l => l.Kind is DiffLineKind.Inserted
+                                   or DiffLineKind.Deleted
+                                   or DiffLineKind.Modified)
+                .ToList();
+            var previewLines = changedLines
+                .Take(3)
+                .Select(l => (l.Kind switch
+                {
+                    DiffLineKind.Inserted => "+ ",
+                    DiffLineKind.Deleted => "- ",
+                    DiffLineKind.Modified => "~ ",
+                    _ => "  ",
+                }) + l.Text);
             var resp = ConfirmHandler?.Invoke(new ConfirmationRequest(
                 Title: "Revert hunk?",
                 Message: "Discard this hunk from the working tree? This cannot be undone via git.\n\n"
-                       + "Preview:\n" + string.Join("\n", preview)
-                       + (inputs.Hunk.Lines.Count > 3 ? "\n…" : ""),
+                       + "Preview:\n" + string.Join("\n", previewLines)
+                       + (changedLines.Count > 3 ? "\n…" : ""),
                 ConfirmText: "Revert",
                 CancelText: "Cancel",
                 ShowDontAskAgain: true));