refactor: split MainViewModel core under 500 lines — 500-limit Phase 3, Commit A

Core MainViewModel.cs 758 -> 495 lines. Moved the source add/remove/image
block (AddSource, NextSourceName, AddImage, BuildImageCandidates,
PickImageBytes, AddAsset, AddImageSource, AddReusedImage, RemoveElement) into a
new MainViewModel.Sources.cs partial, and WebView2 source hosting
(RegisterLoadedWebSources, InitWebView2, OnWebView2PreviewBitmapChanged) into a
new MainViewModel.Web.cs partial. Pruned now-dead usings from core.

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.Sources.cs

@@ -0,0 +1,251 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Windows;
+using Microsoft.Win32;
+using ytLive.Helpers;
+using ytLive.Models;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.ViewModels;
+
+public partial class MainViewModel
+{
+    private void AddSource(object? parameter)
+    {
+        var scene = StagedScene;
+        if (scene == null) return;
+
+        var sourceType = parameter is SourceType typed
+            ? typed
+            : Enum.TryParse<SourceType>(parameter?.ToString(), true, out var parsed) ? parsed : SourceType.Image;
+        var baseName = sourceType switch
+        {
+            SourceType.Image => "Image",
+            SourceType.TextOverlay => "Text",
+            SourceType.Countdown => "Countdown",
+            SourceType.WebSource => "Web Resource",
+            SourceType.ChatBox => "YouTube Chat",
+            _ => "Layer",
+        };
+
+        // The (+) menu only adds regular layers. The Background is a locked
+        // per-screen layer managed by EnsureBackground/NormalizeBackgrounds —
+        // never user-created, never deleted. YouTube Chat is one-per-layout.
+        if (sourceType is SourceType.DisplayCapture or SourceType.WindowCapture or SourceType.Background)
+            return;
+        if (sourceType == SourceType.ChatBox && !CanAddYouTubeChat)
+            return;
+
+        // Web resources use "Web Resource-N" naming (0-indexed per scene)
+        var webCount = scene.Elements.OfType<Source>().Count(s => s.Type == SourceType.WebSource);
+        var source = new Source
+        {
+            Name = sourceType == SourceType.WebSource
+                ? $"{baseName}-{webCount}"
+                : NextSourceName(scene, baseName),
+            Type = sourceType,
+            X = sourceType == SourceType.ChatBox ? 1400 : 0,
+            Y = sourceType == SourceType.ChatBox ? 20 : 0,
+            Width = sourceType == SourceType.WebSource ? 640 : sourceType == SourceType.ChatBox ? 500 : 0,
+            Height = sourceType == SourceType.WebSource ? 480 : sourceType == SourceType.ChatBox ? 648 : 0,
+        };
+        scene.Elements.Add(source);
+
+        if (sourceType == SourceType.WebSource)
+        {
+            _webView2Manager?.Register(source);
+            source.PropertyChanged += (_, e) =>
+            {
+                if (e.PropertyName == nameof(Source.WebUri))
+                    _webView2Manager?.OnWebUriChanged(source);
+            };
+        }
+
+        if (sourceType == SourceType.ChatBox)
+        {
+            source.PropertyChanged += (_, e) =>
+            {
+                if (e.PropertyName is nameof(Source.ChatFontSize) or nameof(Source.ChatFontColor)
+                    or nameof(Source.ChatBackgroundColor) or nameof(Source.ChatBackgroundOpacity)
+                    or nameof(Source.ChatMaxMessages) or nameof(Source.ChatShowTimestamps)
+                    or nameof(Source.ChatShowBadges) or nameof(Source.ChatMemberColor)
+                    or nameof(Source.ChatSuperChatColor) or nameof(Source.ChatFontFamily)
+                    or nameof(Source.ChatFadeSeconds) or nameof(Source.Width) or nameof(Source.Height))
+                    UpdateChatBoxPreview(source);
+            };
+            UpdateChatBoxPreview(source);
+        }
+
+        OnPropertyChanged(nameof(ShowEmptySceneHint));
+        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
+        UpdateActiveBackground();
+    }
+
+    // Duplicate resource names get an incrementing suffix with no space: Image,
+    // Image2, Image3… The next free number is derived from the names actually in
+    // the scene, so deleting a middle resource never collides with a survivor.
+    private static string NextSourceName(Scene scene, string baseName)
+    {
+        var taken = scene.Elements.OfType<Source>()
+            .Select(s => s.Name)
+            .Where(n => string.Equals(n, baseName, StringComparison.OrdinalIgnoreCase)
+                || (n.Length > baseName.Length
+                    && n.StartsWith(baseName, StringComparison.OrdinalIgnoreCase)
+                    && int.TryParse(n.Substring(baseName.Length), out _)))
+            .ToHashSet(StringComparer.OrdinalIgnoreCase);
+        if (!taken.Contains(baseName)) return baseName;
+        for (var i = 2; ; i++)
+            if (!taken.Contains($"{baseName}{i}"))
+                return $"{baseName}{i}";
+    }
+
+    private void AddImage()
+    {
+        var scene = StagedScene;
+        if (scene == null) return;
+
+        var candidates = BuildImageCandidates();
+        if (candidates.Count == 0)
+        {
+            var bytes = PickImageBytes("Choose an image");
+            if (bytes != null) AddImageSource(bytes);
+            return;
+        }
+
+        var dialog = new ReuseImageDialog(new ReuseImageViewModel(candidates))
+        {
+            Owner = Application.Current.MainWindow
+        };
+        if (dialog.ShowDialog() != true) return;
+
+        if (dialog.WantsNew)
+        {
+            var bytes = PickImageBytes("Choose an image");
+            if (bytes != null) AddImageSource(bytes);
+        }
+        else
+        {
+            AddReusedImage(dialog.PickedAssetId!);
+        }
+    }
+
+    private List<ReuseImageCandidate> BuildImageCandidates()
+    {
+        var candidates = new List<ReuseImageCandidate>();
+        foreach (var scene in Scenes)
+        foreach (var source in scene.Elements.OfType<Source>().Where(s => s.Type == SourceType.Image && !string.IsNullOrWhiteSpace(s.AssetId)))
+        {
+            candidates.Add(new ReuseImageCandidate
+            {
+                SceneName = scene.Name,
+                SourceName = source.Name,
+                AssetId = source.AssetId!
+            });
+        }
+        return candidates;
+    }
+
+    private byte[]? PickImageBytes(string title)
+    {
+        var dialog = new OpenFileDialog
+        {
+            Title = title,
+            Filter = "Image files (*.png;*.jpg;*.jpeg;*.bmp;*.gif;*.webp)|*.png;*.jpg;*.jpeg;*.bmp;*.gif;*.webp|All files (*.*)|*.*"
+        };
+        if (dialog.ShowDialog() != true) return null;
+        try
+        {
+            return File.ReadAllBytes(dialog.FileName);
+        }
+        catch (Exception ex)
+        {
+            _notifications.Warning("Couldn't read that image", ex.Message);
+            return null;
+        }
+    }
+
+    private string? AddAsset(byte[] bytes)
+    {
+        try
+        {
+            var image = ImageCache.FromBytes(bytes);
+            var id = _layoutStore.UpsertAsset(bytes, image?.PixelWidth ?? 0, image?.PixelHeight ?? 0);
+            if (id != null && image != null) ImageCache.Put(id, image);
+            return id;
+        }
+        catch (Exception ex)
+        {
+            Debug.WriteLine($"Asset store failed: {ex.Message}");
+            return null;
+        }
+    }
+
+    private void AddImageSource(byte[] bytes)
+    {
+        if (bytes.Length == 0) return;
+        var assetId = AddAsset(bytes);
+        if (assetId != null) AddReusedImage(assetId);
+    }
+
+    private void AddReusedImage(string assetId)
+    {
+        var scene = StagedScene;
+        if (scene == null || string.IsNullOrWhiteSpace(assetId)) return;
+
+        var source = new Source { Name = NextSourceName(scene, "Image"), Type = SourceType.Image, AssetId = assetId };
+
+        var image = ImageCache.Get(assetId);
+        if (image != null)
+        {
+            var scale = Math.Min(640.0 / image.PixelWidth, 480.0 / image.PixelHeight);
+            source.Width = image.PixelWidth * scale;
+            source.Height = image.PixelHeight * scale;
+            source.X = (1920 - source.Width) / 2;
+            source.Y = (1080 - source.Height) / 2;
+        }
+
+        scene.Elements.Add(source);
+        SelectedElement = source;
+        OnPropertyChanged(nameof(ShowEmptySceneHint));
+        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
+        UpdateActiveBackground();
+    }
+
+    // Removes an element from the active scene. For a webcam config this drops the
+    // scene's usage and releases one camera reference (CameraManager stops the
+    // session when the last using scene lets go). When the last config anywhere is
+    // removed, the webcam identity is cleared too — re-adding opens the picker
+    // again instead of silently resurrecting the old camera.
+    private void RemoveElement(SceneElement? element)
+    {
+        var scene = StagedScene;
+        if (scene == null || element == null) return;
+        if (element is Source { IsBackground: true }) return;
+        if (element is WebcamSceneConfig && _webcam != null)
+        {
+            if (SelectedElement == element)
+                SelectedElement = null;
+            _ = _cameraManager.ReleaseAsync(_webcam.DeviceId);
+        }
+        if (element is Source { Type: SourceType.WebSource } webSource)
+            _webView2Manager?.Unregister(webSource.Id);
+        scene.Elements.Remove(element);
+
+        if (element is WebcamSceneConfig && _webcam != null
+            && !Scenes.Any(s => s.Elements.OfType<WebcamSceneConfig>().Any()))
+        {
+            _webcam = null;
+            OnPropertyChanged(nameof(CanChangeWebcam));
+            OnPropertyChanged(nameof(CanAddWebcam));
+            OnPropertyChanged(nameof(CanShowWebcamInStagedScene));
+        }
+
+        OnPropertyChanged(nameof(ShowEmptySceneHint));
+        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
+        UpdateActiveBackground();
+    }
+}

ViewModels/MainViewModel.Web.cs

@@ -0,0 +1,41 @@
+using System.Linq;
+using System.Windows.Controls;
+using System.Windows.Media.Imaging;
+using ytLive.Models;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.ViewModels;
+
+public partial class MainViewModel
+{
+    private void RegisterLoadedWebSources()
+    {
+        if (_webView2Manager == null) return;
+
+        foreach (var source in Scenes.SelectMany(s => s.Elements).OfType<Source>()
+                     .Where(s => s.Type == SourceType.WebSource))
+        {
+            _webView2Manager.Register(source);
+            source.PropertyChanged += (_, e) =>
+            {
+                if (e.PropertyName == nameof(Source.WebUri))
+                    _webView2Manager.OnWebUriChanged(source);
+            };
+        }
+    }
+
+    public void InitWebView2(Panel hostPanel)
+    {
+        _webView2Manager = new WebView2Manager(hostPanel, System.Windows.Application.Current.Dispatcher);
+        _webView2Manager.PreviewBitmapChanged += OnWebView2PreviewBitmapChanged;
+        RegisterLoadedWebSources();
+    }
+
+    private void OnWebView2PreviewBitmapChanged(string sourceId, WriteableBitmap bitmap)
+    {
+        foreach (var scene in Scenes)
+            foreach (var el in scene.Elements.OfType<Source>().Where(s => s.Id == sourceId))
+                el.VideoImageSource = bitmap;
+    }
+}

ViewModels/MainViewModel.cs

@@ -1,5 +1,3 @@
-using System.Collections.ObjectModel;
-using System.Collections.Specialized;
 using System.ComponentModel;
 using System.Diagnostics;
 using System.IO;
@@ -8,10 +6,7 @@ using System.Windows.Controls;
 using System.Windows.Input;
 using System.Windows.Interop;
 using System.Windows.Media;
-using System.Windows.Media.Imaging;
 using System.Windows.Threading;
-using Microsoft.Win32;
-using NAudio.CoreAudioApi;
 using ytLive.Helpers;
 using ytLive.Models;
 using ytLive.Services;
@@ -383,35 +378,6 @@ public partial class MainViewModel : ViewModelBase
         AppLog.Write("LoadLayout end");
     }
 
-    private void RegisterLoadedWebSources()
-    {
-        if (_webView2Manager == null) return;
-
-        foreach (var source in Scenes.SelectMany(s => s.Elements).OfType<Source>()
-                     .Where(s => s.Type == SourceType.WebSource))
-        {
-            _webView2Manager.Register(source);
-            source.PropertyChanged += (_, e) =>
-            {
-                if (e.PropertyName == nameof(Source.WebUri))
-                    _webView2Manager.OnWebUriChanged(source);
-            };
-        }
-    }
-
-    public void InitWebView2(Panel hostPanel)
-    {
-        _webView2Manager = new WebView2Manager(hostPanel, System.Windows.Application.Current.Dispatcher);
-        _webView2Manager.PreviewBitmapChanged += OnWebView2PreviewBitmapChanged;
-        RegisterLoadedWebSources();
-    }
-
-    private void OnWebView2PreviewBitmapChanged(string sourceId, WriteableBitmap bitmap)
-    {
-        foreach (var scene in Scenes)
-            foreach (var el in scene.Elements.OfType<Source>().Where(s => s.Id == sourceId))
-                el.VideoImageSource = bitmap;
-    }
 
     public void Shutdown()
     {
@@ -463,240 +429,6 @@ public partial class MainViewModel : ViewModelBase
             OnPropertyChanged(nameof(BackgroundVisible));
     }
 
-    private void AddSource(object? parameter)
-    {
-        var scene = StagedScene;
-        if (scene == null) return;
-
-        var sourceType = parameter is SourceType typed
-            ? typed
-            : Enum.TryParse<SourceType>(parameter?.ToString(), true, out var parsed) ? parsed : SourceType.Image;
-        var baseName = sourceType switch
-        {
-            SourceType.Image => "Image",
-            SourceType.TextOverlay => "Text",
-            SourceType.Countdown => "Countdown",
-            SourceType.WebSource => "Web Resource",
-            SourceType.ChatBox => "YouTube Chat",
-            _ => "Layer",
-        };
-
-        // The (+) menu only adds regular layers. The Background is a locked
-        // per-screen layer managed by EnsureBackground/NormalizeBackgrounds —
-        // never user-created, never deleted. YouTube Chat is one-per-layout.
-        if (sourceType is SourceType.DisplayCapture or SourceType.WindowCapture or SourceType.Background)
-            return;
-        if (sourceType == SourceType.ChatBox && !CanAddYouTubeChat)
-            return;
-
-        // Web resources use "Web Resource-N" naming (0-indexed per scene)
-        var webCount = scene.Elements.OfType<Source>().Count(s => s.Type == SourceType.WebSource);
-        var source = new Source
-        {
-            Name = sourceType == SourceType.WebSource
-                ? $"{baseName}-{webCount}"
-                : NextSourceName(scene, baseName),
-            Type = sourceType,
-            X = sourceType == SourceType.ChatBox ? 1400 : 0,
-            Y = sourceType == SourceType.ChatBox ? 20 : 0,
-            Width = sourceType == SourceType.WebSource ? 640 : sourceType == SourceType.ChatBox ? 500 : 0,
-            Height = sourceType == SourceType.WebSource ? 480 : sourceType == SourceType.ChatBox ? 648 : 0,
-        };
-        scene.Elements.Add(source);
-
-        if (sourceType == SourceType.WebSource)
-        {
-            _webView2Manager?.Register(source);
-            source.PropertyChanged += (_, e) =>
-            {
-                if (e.PropertyName == nameof(Source.WebUri))
-                    _webView2Manager?.OnWebUriChanged(source);
-            };
-        }
-
-        if (sourceType == SourceType.ChatBox)
-        {
-            source.PropertyChanged += (_, e) =>
-            {
-                if (e.PropertyName is nameof(Source.ChatFontSize) or nameof(Source.ChatFontColor)
-                    or nameof(Source.ChatBackgroundColor) or nameof(Source.ChatBackgroundOpacity)
-                    or nameof(Source.ChatMaxMessages) or nameof(Source.ChatShowTimestamps)
-                    or nameof(Source.ChatShowBadges) or nameof(Source.ChatMemberColor)
-                    or nameof(Source.ChatSuperChatColor) or nameof(Source.ChatFontFamily)
-                    or nameof(Source.ChatFadeSeconds) or nameof(Source.Width) or nameof(Source.Height))
-                    UpdateChatBoxPreview(source);
-            };
-            UpdateChatBoxPreview(source);
-        }
-
-        OnPropertyChanged(nameof(ShowEmptySceneHint));
-        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
-        UpdateActiveBackground();
-    }
-
-    // Duplicate resource names get an incrementing suffix with no space: Image,
-    // Image2, Image3… The next free number is derived from the names actually in
-    // the scene, so deleting a middle resource never collides with a survivor.
-    private static string NextSourceName(Scene scene, string baseName)
-    {
-        var taken = scene.Elements.OfType<Source>()
-            .Select(s => s.Name)
-            .Where(n => string.Equals(n, baseName, StringComparison.OrdinalIgnoreCase)
-                || (n.Length > baseName.Length
-                    && n.StartsWith(baseName, StringComparison.OrdinalIgnoreCase)
-                    && int.TryParse(n.Substring(baseName.Length), out _)))
-            .ToHashSet(StringComparer.OrdinalIgnoreCase);
-        if (!taken.Contains(baseName)) return baseName;
-        for (var i = 2; ; i++)
-            if (!taken.Contains($"{baseName}{i}"))
-                return $"{baseName}{i}";
-    }
-
-    private void AddImage()
-    {
-        var scene = StagedScene;
-        if (scene == null) return;
-
-        var candidates = BuildImageCandidates();
-        if (candidates.Count == 0)
-        {
-            var bytes = PickImageBytes("Choose an image");
-            if (bytes != null) AddImageSource(bytes);
-            return;
-        }
-
-        var dialog = new ReuseImageDialog(new ReuseImageViewModel(candidates))
-        {
-            Owner = Application.Current.MainWindow
-        };
-        if (dialog.ShowDialog() != true) return;
-
-        if (dialog.WantsNew)
-        {
-            var bytes = PickImageBytes("Choose an image");
-            if (bytes != null) AddImageSource(bytes);
-        }
-        else
-        {
-            AddReusedImage(dialog.PickedAssetId!);
-        }
-    }
-
-    private List<ReuseImageCandidate> BuildImageCandidates()
-    {
-        var candidates = new List<ReuseImageCandidate>();
-        foreach (var scene in Scenes)
-        foreach (var source in scene.Elements.OfType<Source>().Where(s => s.Type == SourceType.Image && !string.IsNullOrWhiteSpace(s.AssetId)))
-        {
-            candidates.Add(new ReuseImageCandidate
-            {
-                SceneName = scene.Name,
-                SourceName = source.Name,
-                AssetId = source.AssetId!
-            });
-        }
-        return candidates;
-    }
-
-    private byte[]? PickImageBytes(string title)
-    {
-        var dialog = new OpenFileDialog
-        {
-            Title = title,
-            Filter = "Image files (*.png;*.jpg;*.jpeg;*.bmp;*.gif;*.webp)|*.png;*.jpg;*.jpeg;*.bmp;*.gif;*.webp|All files (*.*)|*.*"
-        };
-        if (dialog.ShowDialog() != true) return null;
-        try
-        {
-            return File.ReadAllBytes(dialog.FileName);
-        }
-        catch (Exception ex)
-        {
-            _notifications.Warning("Couldn't read that image", ex.Message);
-            return null;
-        }
-    }
-
-    private string? AddAsset(byte[] bytes)
-    {
-        try
-        {
-            var image = ImageCache.FromBytes(bytes);
-            var id = _layoutStore.UpsertAsset(bytes, image?.PixelWidth ?? 0, image?.PixelHeight ?? 0);
-            if (id != null && image != null) ImageCache.Put(id, image);
-            return id;
-        }
-        catch (Exception ex)
-        {
-            Debug.WriteLine($"Asset store failed: {ex.Message}");
-            return null;
-        }
-    }
-
-    private void AddImageSource(byte[] bytes)
-    {
-        if (bytes.Length == 0) return;
-        var assetId = AddAsset(bytes);
-        if (assetId != null) AddReusedImage(assetId);
-    }
-
-    private void AddReusedImage(string assetId)
-    {
-        var scene = StagedScene;
-        if (scene == null || string.IsNullOrWhiteSpace(assetId)) return;
-
-        var source = new Source { Name = NextSourceName(scene, "Image"), Type = SourceType.Image, AssetId = assetId };
-
-        var image = ImageCache.Get(assetId);
-        if (image != null)
-        {
-            var scale = Math.Min(640.0 / image.PixelWidth, 480.0 / image.PixelHeight);
-            source.Width = image.PixelWidth * scale;
-            source.Height = image.PixelHeight * scale;
-            source.X = (1920 - source.Width) / 2;
-            source.Y = (1080 - source.Height) / 2;
-        }
-
-        scene.Elements.Add(source);
-        SelectedElement = source;
-        OnPropertyChanged(nameof(ShowEmptySceneHint));
-        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
-        UpdateActiveBackground();
-    }
-
-    // Removes an element from the active scene. For a webcam config this drops the
-    // scene's usage and releases one camera reference (CameraManager stops the
-    // session when the last using scene lets go). When the last config anywhere is
-    // removed, the webcam identity is cleared too — re-adding opens the picker
-    // again instead of silently resurrecting the old camera.
-    private void RemoveElement(SceneElement? element)
-    {
-        var scene = StagedScene;
-        if (scene == null || element == null) return;
-        if (element is Source { IsBackground: true }) return;
-        if (element is WebcamSceneConfig && _webcam != null)
-        {
-            if (SelectedElement == element)
-                SelectedElement = null;
-            _ = _cameraManager.ReleaseAsync(_webcam.DeviceId);
-        }
-        if (element is Source { Type: SourceType.WebSource } webSource)
-            _webView2Manager?.Unregister(webSource.Id);
-        scene.Elements.Remove(element);
-
-        if (element is WebcamSceneConfig && _webcam != null
-            && !Scenes.Any(s => s.Elements.OfType<WebcamSceneConfig>().Any()))
-        {
-            _webcam = null;
-            OnPropertyChanged(nameof(CanChangeWebcam));
-            OnPropertyChanged(nameof(CanAddWebcam));
-            OnPropertyChanged(nameof(CanShowWebcamInStagedScene));
-        }
-
-        OnPropertyChanged(nameof(ShowEmptySceneHint));
-        OnPropertyChanged(nameof(ShowSourcesEmptyHint));
-        UpdateActiveBackground();
-    }
 
     // Scene-element → latest frame, for the live compositor. The map mirrors the
     // preview: webcam by DeviceId, live captures (the background) by CaptureKey,

ViewModels/index.md

@@ -62,5 +62,10 @@ Progress (Commit N/12 · tag):
 - [x] **Commit 17 · `refactor-commit-17`** — **LeftPanel** → `Controls/LeftPanel.xaml(.cs)` (Row 1 Col 0 left panel: layers/properties edit UI + live chat list). Moved ALL property-editor and chat handlers: `SourceNameBox_*`/`CommitSourceEdit`, `AddSourceButton_Click`, `OnChatMessagesChanged` (now self-subscribed on Loaded), `MirrorButton_Click`, `ShapeButton_Click`, `SourceMenu_Remove`, `BackgroundMenu_*`, `OpacitySlider_ValueChanged`, list drag-to-reorder (`_dragStartPoint`+`List_Preview*`+`EndListDrag`+`FindItemContainerAt`), and the snapshot family (`TakeSnapshot`, `WebcamBorder*`, `Countdown*`, `Web*`). Window exposes via `_leftPanel.IsClickInside(original)` (deselect guard) and `_leftPanel.OnSelectionChanged(vm)` (opacity label + snapshot). Pending: overlay host region.
 - [x] **Commit 18 · `refactor-commit-18`** — **OverlayHost** → `Controls/OverlayHost.xaml(.cs)` (settings/bug/feature/about dialogs). `BackToAbout_Click` moved with it (uses inherited `DataContext`). Window keeps the thin `WebViewHostPanel` + `ToastArea` root stubs so name lookups are untouched (WebView2 ctor init, `NotificationService.InArea("ToastArea")`) — toast/WebView2 behavior unchanged. **Phase 2 COMPLETE** — MainWindow.xaml carved down to a 123-line shell over 6 controls (TopBar, HealthBanner, BottomBar, SceneThumbnailStrip, LeftPanel, PreviewPane, OverlayHost). Pending: session wrap-up (HANDOFF.md) + push on approval.
 
+**Phase 3 — 500-line compliance (production `.cs` only).** Bring all production
+source files under 500 lines. One roll-back commit per file; same drill
+(0-warning build, `verify.sh` 246+2, explicit-file scope-check, tracker in same
+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.
 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.