Add Git overlay, operation tools, and virtualized preview so large folders stay responsive.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Explorer.Application.Tests/ThumbnailSchedulerTests.cs

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+using Explorer.Application;
+
+namespace Explorer.Application.Tests;
+
+public class ThumbnailSchedulerTests
+{
+    [Fact]
+    public void Only_window_items_are_queued_not_the_whole_directory()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        var window = Enumerable.Range(0, 40)
+            .Select(i => Job($@"C:\v{i}.jpg", rank: 0))
+            .Concat(Enumerable.Range(0, 24).Select(i => Job($@"C:\p{i}.jpg", rank: 1)))
+            .ToList();
+        scheduler.SetWindow(1, window);
+
+        var taken = Drain(scheduler);
+        Assert.Equal(64, taken.Count);
+        Assert.DoesNotContain(taken, j => j.Key.Path.Contains(@"\rest", StringComparison.Ordinal));
+        Assert.True(taken.Take(40).All(j => j.Rank == 0));
+        Assert.Equal(0, scheduler.InFlight);
+    }
+
+    [Fact]
+    public void Visible_jobs_are_taken_before_prefetch()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        scheduler.SetWindow(1,
+        [
+            Job(@"C:\far.jpg", rank: 1),
+            Job(@"C:\near.jpg", rank: 0)
+        ]);
+
+        Assert.True(scheduler.TryTake(out var first));
+        Assert.Equal(@"C:\near.jpg", first.Key.Path);
+        Assert.True(scheduler.TryTake(out var second));
+        Assert.Equal(@"C:\far.jpg", second.Key.Path);
+        scheduler.Complete(first, generated: true, failed: false);
+        scheduler.Complete(second, generated: true, failed: false);
+    }
+
+    [Fact]
+    public void Scrolling_away_drops_obsolete_pending_work()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        scheduler.SetWindow(1, [Job(@"C:\old.jpg")]);
+        scheduler.SetWindow(1, [Job(@"C:\new.jpg")]);
+
+        Assert.True(scheduler.TryTake(out var job));
+        Assert.Equal(@"C:\new.jpg", job.Key.Path);
+        Assert.False(scheduler.TryTake(out _));
+        Assert.True(scheduler.Snapshot().Cancelled > 0);
+        scheduler.Complete(job, generated: true, failed: false);
+    }
+
+    [Fact]
+    public void Navigation_generation_invalidates_previous_jobs()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        scheduler.SetWindow(1, [Job(@"C:\a.jpg")]);
+        Assert.True(scheduler.TryTake(out var stale));
+        scheduler.Reset(2);
+        Assert.False(scheduler.IsCurrent(stale));
+        Assert.False(scheduler.TryTake(out _));
+        scheduler.SetWindow(2, [Job(@"C:\b.jpg", generation: 2)]);
+        Assert.True(scheduler.TryTake(out var next));
+        Assert.Equal(@"C:\b.jpg", next.Key.Path);
+        scheduler.Complete(next, generated: true, failed: false);
+    }
+
+    [Fact]
+    public void Cache_hit_skips_regeneration()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        var key = ThumbnailKey.From(@"C:\hit.jpg", DateTimeOffset.UnixEpoch, 128);
+        scheduler.MarkCached(key);
+        scheduler.SetWindow(1, [new ThumbnailJob(key, 0, 1)]);
+        Assert.False(scheduler.TryTake(out _));
+        scheduler.RecordCacheHit();
+        Assert.Equal(1, scheduler.Snapshot().CacheHits);
+    }
+
+    [Fact]
+    public void Window_is_capped_so_the_queue_cannot_hold_the_whole_directory()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        var huge = Enumerable.Range(0, 500).Select(i => Job($@"C:\{i}.jpg")).ToList();
+        scheduler.SetWindow(1, huge);
+        Assert.True(scheduler.Snapshot().Window <= ThumbnailScheduler.MaxPending);
+        var taken = Drain(scheduler);
+        Assert.Equal(ThumbnailScheduler.MaxPending, taken.Count);
+    }
+
+    [Fact]
+    public void In_flight_work_is_tracked_until_complete()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        scheduler.SetWindow(1, Enumerable.Range(0, 8).Select(i => Job($@"C:\{i}.jpg")).ToList());
+        Assert.True(scheduler.TryTake(out var a));
+        Assert.True(scheduler.TryTake(out var b));
+        Assert.Equal(2, scheduler.InFlight);
+        scheduler.Complete(a, generated: true, failed: false);
+        Assert.Equal(1, scheduler.InFlight);
+        scheduler.Complete(b, generated: true, failed: false);
+        Assert.Equal(0, scheduler.InFlight);
+    }
+
+    [Fact]
+    public void Preview_disabled_cancels_window()
+    {
+        var scheduler = new ThumbnailScheduler();
+        scheduler.Reset(1);
+        scheduler.SetEnabled(true);
+        scheduler.SetWindow(1, [Job(@"C:\x.jpg")]);
+        scheduler.SetEnabled(false);
+        Assert.False(scheduler.TryTake(out _));
+    }
+
+    [Fact]
+    public void Cache_key_changes_when_file_is_modified()
+    {
+        var first = ThumbnailKey.From(@"C:\a.jpg", DateTimeOffset.UnixEpoch, 128);
+        var second = ThumbnailKey.From(@"C:\a.jpg", DateTimeOffset.UnixEpoch.AddHours(1), 128);
+        Assert.NotEqual(first, second);
+    }
+
+    [Fact]
+    public void Lru_cache_evicts_oldest_when_full()
+    {
+        var cache = new LruCache<string, int>(2);
+        cache.Set("a", 1);
+        cache.Set("b", 2);
+        cache.Set("c", 3);
+        Assert.False(cache.TryGet("a", out _));
+        Assert.True(cache.TryGet("b", out var b));
+        Assert.Equal(2, b);
+        cache.Set("d", 4);
+        Assert.False(cache.TryGet("c", out _));
+        Assert.True(cache.TryGet("b", out _));
+    }
+
+    private static ThumbnailJob Job(string path, int rank = 0, int generation = 1)
+        => new(ThumbnailKey.From(path, DateTimeOffset.UnixEpoch, 128), rank, generation);
+
+    private static List<ThumbnailJob> Drain(ThumbnailScheduler scheduler)
+    {
+        var taken = new List<ThumbnailJob>();
+        while (scheduler.TryTake(out var job))
+        {
+            taken.Add(job);
+            scheduler.Complete(job, generated: true, failed: false);
+        }
+
+        return taken;
+    }
+}