DiffViewer: add test coverage for JumpToHunk during in-flight reload (stale bar click)

[geevensingh]

Problem

DiffPaneViewModel.JumpToHunk(int index) (DiffViewer/ViewModels/DiffPaneViewModel.cs:647-659) is the entry point used by the hunk overview bar when the user clicks a marker:

public void JumpToHunk(int index)
{
    if (index < 0 || index >= _currentHunks.Count) return;
    CurrentHunkIndex = index;
    RaiseHunkNav();
}

The clamp is documented (XML doc lines 647-653):

Out-of-range indices are clamped to a no-op rather than throwing — the overview bar's hit-test math can briefly disagree with the VM's hunk list mid-reload.

This is the right behaviour: during a load, the bar's geometry was computed against the previous hunk list, and a click can resolve to an index that no longer exists. But there is no test for this scenario, and:

1. If someone removes the bounds check thinking "the bar only emits valid indices," any user click during a reload will throw IndexOutOfRangeException and crash the app.
2. If someone changes the no-op to instead clamp index to [0, _currentHunks.Count - 1], the user will be silently scrolled to a different hunk than they clicked — a subtle UX bug.

Suggested tests

Add to DiffPaneViewModelTests.cs:

[Fact]
public void JumpToHunk_OutOfRange_NoOps()
{
    var repo = new FakeRepoForKeyboard();
    repo.SetChanges(ModifiedChange("a.cs"));
    repo.LeftText  = "alpha\n";
    repo.RightText = "ALPHA\n";

    var diff = new DiffService();
    var vm = new DiffPaneViewModel(repo, diff);
    vm.LoadAsync(EntryFor("a.cs")).GetAwaiter().GetResult();
    var nHunks = vm.CurrentHunks.Count;

    var raised = 0;
    vm.HunkNavigationRequested += (_, _) => raised++;

    vm.JumpToHunk(-1);            // negative
    vm.JumpToHunk(nHunks);        // one past end
    vm.JumpToHunk(nHunks + 100);  // way past end

    raised.Should().Be(0, "out-of-range JumpToHunk should be a no-op, not throw or clamp");
    vm.CurrentHunkIndex.Should().Be(-1, "CurrentHunkIndex should not be modified");
}

[Fact]
public async Task JumpToHunk_DuringReload_DropsStaleClick()
{
    // Simulates the documented race: bar's hit-test resolved to an index
    // valid for the OLD hunk list, but ApplyResult has since installed
    // a smaller list (or empty list during placeholder transition).
    var repo = new FakeRepoForKeyboard();
    repo.SetChanges(ModifiedChange("a.cs"));
    repo.LeftText  = "a\nb\nc\nd\ne\nf\n";       // 6 hunks worth of variation
    repo.RightText = "A\nb\nC\nd\nE\nf\n";

    var diff = new DiffService();
    var vm = new DiffPaneViewModel(repo, diff);
    await vm.LoadAsync(EntryFor("a.cs"));
    var staleMaxIndex = vm.CurrentHunks.Count - 1;

    // Now load an empty diff (e.g., user toggled IgnoreWhitespace and the
    // file becomes whitespace-only). CurrentHunks goes empty.
    repo.LeftText = repo.RightText = "same\n";
    await vm.LoadAsync(EntryFor("a.cs"));

    Action click = () => vm.JumpToHunk(staleMaxIndex);
    click.Should().NotThrow("a stale-bar click during reload must not crash");
    vm.CurrentHunkIndex.Should().Be(-1);
}

Citations

• DiffViewer/ViewModels/DiffPaneViewModel.cs:647-659 — JumpToHunk and its docs
• DiffViewer/Rendering/HunkOverviewBar.cs — the producer of JumpToHunk(...) calls
• DiffViewer/ViewModels/DiffPaneViewModel.cs:483-513 — ApplyResult (where _currentHunks is replaced; the race window)

Severity

Low (no known bug today). Test gap protecting against crash on user click during reload — unobservable in normal use, but not tested anywhere.