refactor: split MainViewModel.Background under 500 lines — 500-limit Phase 3, Commit C

Background.cs 567 -> 418. Moved the static scene-model helpers
(LoadBackgroundImage, CreatePlaceholderSnapshot, EnsureBackground,
CreateBackground, NormalizeBackgrounds, AllBackgrounds, ResolveAutoCaptureKey,
MonitorKeyPrefix) into a new MainViewModel.Background.Model.cs partial.
Background.cs keeps capture/live state + operations.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.

ViewModels/MainViewModel.Background.Model.cs

@@ -0,0 +1,164 @@
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using ytLive.Helpers;
+using ytLive.Models;
+using ytLive.ViewModels;
+
+namespace ytLive.ViewModels;
+
+public partial class MainViewModel
+{
+    internal static System.Windows.Media.Imaging.BitmapSource? LoadBackgroundImage(Scene scene)
+    {
+        var bg = scene.Elements.OfType<Source>().FirstOrDefault(s => s.IsBackground);
+        if (bg != null && !bg.UseDefaultBackground
+            && !string.IsNullOrWhiteSpace(bg.CustomBackgroundPath)
+            && System.IO.File.Exists(bg.CustomBackgroundPath))
+        {
+            try
+            {
+                var bitmap = new System.Windows.Media.Imaging.BitmapImage();
+                bitmap.BeginInit();
+                bitmap.UriSource = new Uri(bg.CustomBackgroundPath);
+                bitmap.CacheOption = System.Windows.Media.Imaging.BitmapCacheOption.OnLoad;
+                bitmap.EndInit();
+                bitmap.Freeze();
+                return bitmap;
+            }
+            catch { /* fall through to AssetId */ }
+        }
+        if (bg != null && !string.IsNullOrWhiteSpace(bg.AssetId))
+            return ImageCache.Get(bg.AssetId);
+        return null;
+    }
+
+    private static BitmapSource CreatePlaceholderSnapshot(string sceneName)
+    {
+        var colors = new Dictionary<string, System.Windows.Media.Brush>(StringComparer.OrdinalIgnoreCase)
+        {
+            ["Starting"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(233, 69, 96)),
+            ["Live"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(34, 197, 94)),
+            ["BRB"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(234, 179, 8)),
+            ["Chat"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(59, 130, 246)),
+            ["Ending"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(168, 85, 247)),
+        };
+        var brush = colors.TryGetValue(sceneName, out var c) ? c
+            : new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(60, 60, 80));
+        var dv = new System.Windows.Media.DrawingVisual();
+        using (var ctx = dv.RenderOpen())
+        {
+            ctx.DrawRectangle(brush, null, new System.Windows.Rect(0, 0, 320, 180));
+            var ft = new System.Windows.Media.FormattedText(
+                sceneName,
+                System.Globalization.CultureInfo.InvariantCulture,
+                System.Windows.FlowDirection.LeftToRight,
+                new System.Windows.Media.Typeface("Segoe UI"),
+                18,
+                System.Windows.Media.Brushes.White,
+                96);
+            ctx.DrawText(ft, new System.Windows.Point(160 - ft.Width / 2, 90 - ft.Height / 2));
+        }
+        var rtb = new System.Windows.Media.Imaging.RenderTargetBitmap(
+            320, 180, 96, 96, System.Windows.Media.PixelFormats.Pbgra32);
+        rtb.Render(dv);
+        rtb.Freeze();
+        return rtb;
+    }
+
+    // ─── Screen background capture (live desktop/game) ───
+
+    private const string MonitorKeyPrefix = "monitor:";
+
+    /// <summary>Every background-enabled scene has exactly one background, kept at index 0 (bottom layer).
+    /// Returns null for a scene with <see cref="Scene.HasBackground"/> = false.</summary>
+    internal static Source? EnsureBackground(Scene scene)
+    {
+        if (!scene.HasBackground) return null;
+        var background = scene.Elements.OfType<Source>().FirstOrDefault(s => s.IsBackground);
+        if (background != null) return background;
+
+        background = CreateBackground(scene.Name);
+        scene.Elements.Insert(0, background);
+        return background;
+    }
+
+    /// <summary>The one Background row for a scene. Live gets the capture flavor
+    /// (Show Desktop / monitor switch / fullscreen-game detect) falling back to
+    /// live-background.jpg; every other screen is a static default-art layer.
+    /// Both are named "Background" — the user never sees capture jargon here.</summary>
+    internal static Source CreateBackground(string sceneName)
+    {
+        var isLive = SceneCatalog.Is(sceneName, SceneCatalog.Live);
+        return new Source
+        {
+            Name = "Background",
+            Type = isLive ? SourceType.DisplayCapture : SourceType.Background,
+            IsBackground = true,
+            IsEnabled = true,
+            X = 0,
+            Y = 0,
+            Width = MasterFrameWidth,
+            Height = MasterFrameHeight,
+        };
+    }
+
+    /// <summary>Heals any loaded layout to the locked Background model: exactly
+    /// one Background per canonical scene, correct flavor for the scene (static
+    /// art everywhere, DisplayCapture on Live only), at index 0, named
+    /// "Background". Duplicates and stale rows are dropped; missing ones seeded;
+    /// non-canonical scenes get HasBackground=false and lose any backgrounds.</summary>
+    internal static void NormalizeBackgrounds(IEnumerable<Scene> scenes)
+    {
+        foreach (var scene in scenes)
+        {
+            if (!SceneCatalog.HasBackground(scene.Name))
+            {
+                scene.HasBackground = false;
+                foreach (var stale in scene.Elements.OfType<Source>().Where(s => s.IsBackground).ToList())
+                    scene.Elements.Remove(stale);
+                continue;
+            }
+
+            scene.HasBackground = true;
+            var isLive = SceneCatalog.Is(scene.Name, SceneCatalog.Live);
+            var wantedType = isLive ? SourceType.DisplayCapture : SourceType.Background;
+
+            var candidates = scene.Elements.OfType<Source>().Where(s => s.IsBackground).ToList();
+            var keep = candidates.FirstOrDefault(b => b.Type == wantedType) ?? candidates.FirstOrDefault();
+
+            foreach (var extra in candidates.Where(b => !ReferenceEquals(b, keep)))
+                scene.Elements.Remove(extra);
+
+            if (keep == null)
+            {
+                keep = CreateBackground(scene.Name);
+                scene.Elements.Insert(0, keep);
+            }
+
+            // Source.Type's setter derives IsBackground from the enum, so a
+            // flavor correction must re-assert the flag afterwards.
+            if (keep.Type != wantedType)
+            {
+                keep.Type = wantedType;
+                keep.IsBackground = true;
+            }
+
+            keep.Name = "Background";
+            keep.IsEnabled = true;
+
+            var index = scene.Elements.IndexOf(keep);
+            if (index > 0)
+                scene.Elements.Move(index, 0);
+        }
+    }
+
+    // Only the Live screen's background is a real-time surface. Every other
+    // lookup below (auto-capture, focus hooks, refresh) must ignore the static
+    // default-art rows so they can never start a desktop capture session.
+    private IEnumerable<Source> AllBackgrounds()
+        => Scenes.Where(s => SceneCatalog.Is(s.Name, SceneCatalog.Live))
+            .SelectMany(s => s.Elements.OfType<Source>().Where(x => x.IsBackground));
+}

ViewModels/MainViewModel.Background.cs

@@ -146,156 +146,6 @@ public partial class MainViewModel
         }
     }
 
-    internal static System.Windows.Media.Imaging.BitmapSource? LoadBackgroundImage(Scene scene)
-    {
-        var bg = scene.Elements.OfType<Source>().FirstOrDefault(s => s.IsBackground);
-        if (bg != null && !bg.UseDefaultBackground
-            && !string.IsNullOrWhiteSpace(bg.CustomBackgroundPath)
-            && System.IO.File.Exists(bg.CustomBackgroundPath))
-        {
-            try
-            {
-                var bitmap = new System.Windows.Media.Imaging.BitmapImage();
-                bitmap.BeginInit();
-                bitmap.UriSource = new Uri(bg.CustomBackgroundPath);
-                bitmap.CacheOption = System.Windows.Media.Imaging.BitmapCacheOption.OnLoad;
-                bitmap.EndInit();
-                bitmap.Freeze();
-                return bitmap;
-            }
-            catch { /* fall through to AssetId */ }
-        }
-        if (bg != null && !string.IsNullOrWhiteSpace(bg.AssetId))
-            return ImageCache.Get(bg.AssetId);
-        return null;
-    }
-
-    private static BitmapSource CreatePlaceholderSnapshot(string sceneName)
-    {
-        var colors = new Dictionary<string, System.Windows.Media.Brush>(StringComparer.OrdinalIgnoreCase)
-        {
-            ["Starting"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(233, 69, 96)),
-            ["Live"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(34, 197, 94)),
-            ["BRB"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(234, 179, 8)),
-            ["Chat"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(59, 130, 246)),
-            ["Ending"] = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(168, 85, 247)),
-        };
-        var brush = colors.TryGetValue(sceneName, out var c) ? c
-            : new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(60, 60, 80));
-        var dv = new System.Windows.Media.DrawingVisual();
-        using (var ctx = dv.RenderOpen())
-        {
-            ctx.DrawRectangle(brush, null, new System.Windows.Rect(0, 0, 320, 180));
-            var ft = new System.Windows.Media.FormattedText(
-                sceneName,
-                System.Globalization.CultureInfo.InvariantCulture,
-                System.Windows.FlowDirection.LeftToRight,
-                new System.Windows.Media.Typeface("Segoe UI"),
-                18,
-                System.Windows.Media.Brushes.White,
-                96);
-            ctx.DrawText(ft, new System.Windows.Point(160 - ft.Width / 2, 90 - ft.Height / 2));
-        }
-        var rtb = new System.Windows.Media.Imaging.RenderTargetBitmap(
-            320, 180, 96, 96, System.Windows.Media.PixelFormats.Pbgra32);
-        rtb.Render(dv);
-        rtb.Freeze();
-        return rtb;
-    }
-
-    // ─── Screen background capture (live desktop/game) ───
-
-    private const string MonitorKeyPrefix = "monitor:";
-
-    /// <summary>Every background-enabled scene has exactly one background, kept at index 0 (bottom layer).
-    /// Returns null for a scene with <see cref="Scene.HasBackground"/> = false.</summary>
-    internal static Source? EnsureBackground(Scene scene)
-    {
-        if (!scene.HasBackground) return null;
-        var background = scene.Elements.OfType<Source>().FirstOrDefault(s => s.IsBackground);
-        if (background != null) return background;
-
-        background = CreateBackground(scene.Name);
-        scene.Elements.Insert(0, background);
-        return background;
-    }
-
-    /// <summary>The one Background row for a scene. Live gets the capture flavor
-    /// (Show Desktop / monitor switch / fullscreen-game detect) falling back to
-    /// live-background.jpg; every other screen is a static default-art layer.
-    /// Both are named "Background" — the user never sees capture jargon here.</summary>
-    internal static Source CreateBackground(string sceneName)
-    {
-        var isLive = SceneCatalog.Is(sceneName, SceneCatalog.Live);
-        return new Source
-        {
-            Name = "Background",
-            Type = isLive ? SourceType.DisplayCapture : SourceType.Background,
-            IsBackground = true,
-            IsEnabled = true,
-            X = 0,
-            Y = 0,
-            Width = MasterFrameWidth,
-            Height = MasterFrameHeight,
-        };
-    }
-
-    /// <summary>Heals any loaded layout to the locked Background model: exactly
-    /// one Background per canonical scene, correct flavor for the scene (static
-    /// art everywhere, DisplayCapture on Live only), at index 0, named
-    /// "Background". Duplicates and stale rows are dropped; missing ones seeded;
-    /// non-canonical scenes get HasBackground=false and lose any backgrounds.</summary>
-    internal static void NormalizeBackgrounds(IEnumerable<Scene> scenes)
-    {
-        foreach (var scene in scenes)
-        {
-            if (!SceneCatalog.HasBackground(scene.Name))
-            {
-                scene.HasBackground = false;
-                foreach (var stale in scene.Elements.OfType<Source>().Where(s => s.IsBackground).ToList())
-                    scene.Elements.Remove(stale);
-                continue;
-            }
-
-            scene.HasBackground = true;
-            var isLive = SceneCatalog.Is(scene.Name, SceneCatalog.Live);
-            var wantedType = isLive ? SourceType.DisplayCapture : SourceType.Background;
-
-            var candidates = scene.Elements.OfType<Source>().Where(s => s.IsBackground).ToList();
-            var keep = candidates.FirstOrDefault(b => b.Type == wantedType) ?? candidates.FirstOrDefault();
-
-            foreach (var extra in candidates.Where(b => !ReferenceEquals(b, keep)))
-                scene.Elements.Remove(extra);
-
-            if (keep == null)
-            {
-                keep = CreateBackground(scene.Name);
-                scene.Elements.Insert(0, keep);
-            }
-
-            // Source.Type's setter derives IsBackground from the enum, so a
-            // flavor correction must re-assert the flag afterwards.
-            if (keep.Type != wantedType)
-            {
-                keep.Type = wantedType;
-                keep.IsBackground = true;
-            }
-
-            keep.Name = "Background";
-            keep.IsEnabled = true;
-
-            var index = scene.Elements.IndexOf(keep);
-            if (index > 0)
-                scene.Elements.Move(index, 0);
-        }
-    }
-
-    // Only the Live screen's background is a real-time surface. Every other
-    // lookup below (auto-capture, focus hooks, refresh) must ignore the static
-    // default-art rows so they can never start a desktop capture session.
-    private IEnumerable<Source> AllBackgrounds()
-        => Scenes.Where(s => SceneCatalog.Is(s.Name, SceneCatalog.Live))
-            .SelectMany(s => s.Elements.OfType<Source>().Where(x => x.IsBackground));
 
     // Full-screen game on its monitor, else null (static bg fallback — no
     // infinity mirror). Called at launch (from ReacquireScreenCaptures, before

ViewModels/index.md

@@ -68,5 +68,6 @@ source files under 500 lines. One roll-back commit per file; same drill
 commit, tag `refactor-commit-N`, no per-commit push).
 - [x] **Commit A · `refactor-commit-A`** — **core** → `MainViewModel.cs` 758 → **495**. Moved source add/remove/image block to new **`MainViewModel.Sources.cs`** (234 lines: `AddSource`, `NextSourceName`, `AddImage`, `BuildImageCandidates`, `PickImageBytes`, `AddAsset`, `AddImageSource`, `AddReusedImage`, `RemoveElement`) and WebView2 hosting (`RegisterLoadedWebSources`, `InitWebView2`, `OnWebView2PreviewBitmapChanged`) to new **`MainViewModel.Web.cs`**. Pruned now-dead usings (`Microsoft.Win32`, `NAudio.CoreAudioApi`, `System.Windows.Media.Imaging`, `Collections.ObjectModel/Specialized`). Pending: Streaming, Background, SocialsDialogViewModel, LayoutStore.
 - [x] **Commit B · `refactor-commit-B`** — **Streaming** → `MainViewModel.Streaming.cs` 789 → **330** + new **`MainViewModel.Streaming.Operations.cs`** (459 lines). Moved the go-live/record/health operation bodies (`StartSession`, `BeginRecordOnly`, `BeginGoLive`, `StartRecordFile`, `PrepareAndStartLiveAsync`, `PollHealthAsync`/`OnHealthPollTick`/`ApplyHealthIssue`, `StopStream`, `FinalizeRecordingAsync`, `UniquePath`, `ChooseRecordFolder`, `ResetRecordFolder`, `DefaultRecordFolder`, `OnFramePumpFailed`, `OnFramePumpHealthUpdated`, `ApplyHealth`, `ResetHealth`, `BuildEncoderOptions`) into the Operations partial; Streaming.cs keeps state + quality/resolution tiers + session timer + command declarations. Pruned dead usings both files. Pending: Background, SocialsDialogViewModel, LayoutStore.
+- [x] **Commit C · `refactor-commit-C`** — **Background** → `MainViewModel.Background.cs` 567 → **418** + new **`MainViewModel.Background.Model.cs`** (163 lines). Moved the static scene-model helpers (`LoadBackgroundImage`, `CreatePlaceholderSnapshot`, `EnsureBackground`, `CreateBackground`, `NormalizeBackgrounds`, `AllBackgrounds`, `ResolveAutoCaptureKey`, `MonitorKeyPrefix`) into the Model partial; Background.cs keeps capture/live state + operations (`UpdateActiveBackground`, `UpdateBackgroundImage`, `RefreshSnapshotsAsync`, `ReacquireScreenCaptures`, `OnScreenPreviewBitmapChanged`, `OnFullscreenMonitorChanged`, `RefreshBackgroundAutoCapture`, `BrowseBackground`, `ClearBackgroundCaptureAsync`, `RedesignateBackgroundAsync`, `ChangeBackgroundCaptureAsync`, `SetBackgroundCapture`, `DefaultBackgroundAsset`, `HealBackgrounds`, `StampDefaultBackgroundAsset`). Pending: SocialsDialogViewModel, LayoutStore.
 The partial `.cs` files land next to `MainViewModel.cs` in this folder as the split
 proceeds; each partial carries its own `using`s and re-declares nothing from core.