DiffViewer: replace revert-hunk preview with a one-line summary

The preview block was rendering as plain unstyled text in a generic white WPF dialog (Segoe UI, no syntax colors), so even after filtering to changed lines with +/- markers, the result looked like it came from a different app than DiffViewer. Plumbing the editor color scheme, font family, and font size into a confirmation dialog isn't worth the view-layer surface area when the user just needs to confirm intent.

Drop the line excerpt entirely and replace it with a one-line count summary the user can read at a glance:

    Discard this hunk from the working tree? This cannot be undone via git.

    1 addition, 1 removal at line 47 in MyMethod().

The reported line number is the first actually-changed line in the hunk (first non-context line's NewLineNumber, falling back to OldLineNumber for pure deletions). hunk.NewStartLine would have pointed at the leading context line DiffService prepends to every hunk, putting the reported position a line or two above where the change actually sits. Function context is the unified-diff '@@ ... @@' trailer, included only when DiffService captured one.

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

Author: geevensingh

DiffViewer.Tests/ViewModels/MainViewModelContextMenuTests.cs

@@ -337,13 +337,15 @@ await RunOnUiSyncContextAsync(async vm =>
     }
 
     [Fact]
-    public async Task RevertHunkAtCaret_PreviewShowsOnlyChangedLinesWithMarkers()
+    public async Task RevertHunkAtCaret_PromptDescribesHunkRatherThanPreviewingLines()
     {
-        // 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.
+        // The dialog used to show a 3-line preview from the top of the hunk
+        // - which, because DiffService prepends context lines around every
+        // hunk, almost always rendered three unchanged lines and zero
+        // indication of what was about to be discarded. The replacement is
+        // a one-line summary of additions/removals + line number so the
+        // user knows what they're confirming without dragging editor
+        // settings into a plain System.Windows MessageBox-style dialog.
         var repo = new FakeRepositoryService(_repoRoot)
         {
             LeftText = "alpha\nbeta\n",
@@ -366,10 +368,14 @@ await RunOnUiSyncContextAsync(async vm =>
         }, 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");
+        seen!.Message.Should().Contain("1 addition", "the inserted 'gamma' line should be counted");
+        seen.Message.Should().Contain("1 removal", "the deleted 'beta' line should be counted");
+        seen.Message.Should().Contain("at line 2",
+            "the prompt should pin the change to its line number in the working tree");
+        seen.Message.Should().NotContain("Preview:",
+            "the old line-by-line preview has been removed");
+        seen.Message.Should().NotContain("alpha",
+            "context lines should not leak into the summary");
     }
 
     [Fact]

DiffViewer/ViewModels/MainViewModel.cs

@@ -213,33 +213,10 @@ private async Task RevertHunkAtCaret(HunkActionContext? ctx)
         var suppress = _settingsService?.Current.SuppressRevertHunkConfirmation ?? false;
         if (!suppress)
         {
-            // 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", previewLines)
-                       + (changedLines.Count > 3 ? "\n…" : ""),
+                       + DescribeHunk(inputs.Hunk),
                 ConfirmText: "Revert",
                 CancelText: "Cancel",
                 ShowDontAskAgain: true));
@@ -264,6 +241,38 @@ private async Task OpenInExternalEditorAtLine(LineActionContext? ctx)
         if (!r.Success) ToastHandler?.Invoke(r.ErrorMessage ?? "Editor launch failed.");
     }
 
+    // One-line human summary of a hunk for the "Revert hunk?" prompt.
+    // DiffService never emits DiffLineKind.Modified today (every change is
+    // expressed as a Deleted+Inserted pair), so additions/removals fully
+    // describe what the revert will undo. The reported line number is the
+    // first actually-changed line - not hunk.NewStartLine, which includes
+    // DiffService's leading context lines and would point a line or two
+    // above where the user's change really sits. Function context comes
+    // from the unified-diff "@@ ... @@" trailer when available.
+    private static string DescribeHunk(DiffHunk hunk)
+    {
+        var added = hunk.Lines.Count(l => l.Kind == DiffLineKind.Inserted);
+        var removed = hunk.Lines.Count(l => l.Kind == DiffLineKind.Deleted);
+
+        static string Plural(int n, string singular, string plural)
+            => $"{n} {(n == 1 ? singular : plural)}";
+
+        var parts = new List<string>(2);
+        if (added > 0) parts.Add(Plural(added, "addition", "additions"));
+        if (removed > 0) parts.Add(Plural(removed, "removal", "removals"));
+        var counts = parts.Count > 0 ? string.Join(", ", parts) : "no line changes";
+
+        var firstChange = hunk.Lines.FirstOrDefault(l => l.Kind != DiffLineKind.Context);
+        var lineNum = firstChange?.NewLineNumber
+                      ?? firstChange?.OldLineNumber
+                      ?? hunk.NewStartLine;
+        var location = $"at line {lineNum}";
+        if (!string.IsNullOrWhiteSpace(hunk.FunctionContext))
+            location += $" in {hunk.FunctionContext}";
+
+        return $"{counts} {location}.";
+    }
+
     // ---------------- Keyboard-shortcut commands ----------------
     // Cross-file / cross-section hunk navigation (F7/F8 family).
     // Toggle aliases for toolbar state (Ctrl+W, Ctrl+I, etc.).