Wrap tray identicon in an ICO container so System.Drawing.Icon accepts it

CHANGELOG.md

@@ -17,12 +17,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `LogsDirectory`. The log rotates at 5 MB.
 
 ### Fixed
+- Tray icon initialisation crash, take two
+  (`ArgumentException: Argument 'picture' must be a picture that can be
+  used as a Icon.`): once H.NotifyIcon could read the file (PR #15) it
+  passed the bytes to `new System.Drawing.Icon(stream, size)`, which
+  only accepts ICO-format input. The tray service now wraps the
+  identicon PNG in a minimal ICO container (modern Windows accepts
+  PNG-in-ICO) and writes `%LOCALAPPDATA%\Snipdeck\tray-icon.ico`.
 - Tray icon initialisation crash (`NullReferenceException` inside
   `H.NotifyIcon.ImageExtensions.ToStream(Uri)`): H.NotifyIcon resolves
   `TaskbarIcon.IconSource` by reading `BitmapImage.UriSource`, but the
   tray service was loading the identicon via `BitmapImage.SetSourceAsync`
   from an in-memory stream — pixels populated, URI null. The service now
-  persists the identicon to `%LOCALAPPDATA%\Snipdeck\tray-icon.png` and
+  persists the identicon to `%LOCALAPPDATA%\Snipdeck\tray-icon.ico` and
   loads the `BitmapImage` from that URI, which is the shape H.NotifyIcon
   expects.
 - First-run crash on startup (`RPC_E_WRONG_THREAD` / `0x8001010E`):

src/Snipdeck.App/Services/HNotifyIconTrayService.cs

@@ -1,3 +1,5 @@
+using System.Buffers.Binary;
+
 using H.NotifyIcon;
 
 using Microsoft.UI.Xaml.Controls;
@@ -12,7 +14,8 @@ internal sealed partial class HNotifyIconTrayService : ITrayService
     {
         // Stable seed so the tray icon never changes between runs of Snipdeck.
         private static readonly Guid _iconSeed = Guid.Parse("5147DEC0-0000-0000-0000-000000000001");
-        private const string _trayIconFileName = "tray-icon.png";
+        private const string _trayIconFileName = "tray-icon.ico";
+        private const int _trayIconPixels = 32;
 
         private readonly IPathProvider _paths;
         private TaskbarIcon? _icon;
@@ -37,10 +40,10 @@ public async Task InitialiseAsync()
                 return;
             }
 
-            // H.NotifyIcon resolves IconSource by reading BitmapImage.UriSource,
-            // not the pixel data — a stream-loaded BitmapImage NREs deep inside
-            // the library. Persist the identicon to a stable file path and load
-            // the BitmapImage from that URI instead.
+            // H.NotifyIcon resolves IconSource by reading BitmapImage.UriSource
+            // and then hands the file bytes to System.Drawing.Icon — which only
+            // accepts ICO format. Wrap the identicon PNG in a minimal ICO
+            // container before writing it out.
             var iconPath = await WriteTrayIconFileAsync();
             var image = new BitmapImage(new Uri(iconPath, UriKind.Absolute));
 
@@ -102,13 +105,43 @@ private void RaiseShowRequested()
 
         private async Task<string> WriteTrayIconFileAsync()
         {
-            var bytes = IdenticonService.GeneratePng(_iconSeed, size: 32);
+            var pngBytes = IdenticonService.GeneratePng(_iconSeed, size: _trayIconPixels);
+            var icoBytes = WrapPngAsIco(pngBytes, _trayIconPixels, _trayIconPixels);
             _ = Directory.CreateDirectory(_paths.AppDataDirectory);
             var path = Path.Combine(_paths.AppDataDirectory, _trayIconFileName);
-            await File.WriteAllBytesAsync(path, bytes);
+            await File.WriteAllBytesAsync(path, icoBytes);
             return path;
         }
 
+        // Modern Windows accepts a PNG embedded inside an ICO container —
+        // just a 6-byte ICONDIR header plus a 16-byte ICONDIRENTRY pointing
+        // at the PNG bytes. Format ref: ICONDIR / ICONDIRENTRY on MSDN.
+        private static byte[] WrapPngAsIco(byte[] png, int width, int height)
+        {
+            const int headerSize = 6;
+            const int entrySize = 16;
+            const int dataOffset = headerSize + entrySize;
+
+            var ico = new byte[dataOffset + png.Length];
+            var span = ico.AsSpan();
+
+            // ICONDIR: reserved(0), type(1 = icon), count(1)
+            BinaryPrimitives.WriteUInt16LittleEndian(span[2..4], 1);
+            BinaryPrimitives.WriteUInt16LittleEndian(span[4..6], 1);
+
+            // ICONDIRENTRY: width/height bytes are 0 for >=256, otherwise the literal size.
+            ico[6] = width >= 256 ? (byte)0 : (byte)width;
+            ico[7] = height >= 256 ? (byte)0 : (byte)height;
+            // ico[8] colorCount = 0 (>= 8bpp), ico[9] reserved = 0 — already zero.
+            BinaryPrimitives.WriteUInt16LittleEndian(span[10..12], 1);   // planes
+            BinaryPrimitives.WriteUInt16LittleEndian(span[12..14], 32);  // bits per pixel
+            BinaryPrimitives.WriteUInt32LittleEndian(span[14..18], (uint)png.Length);
+            BinaryPrimitives.WriteUInt32LittleEndian(span[18..22], dataOffset);
+
+            Buffer.BlockCopy(png, 0, ico, dataOffset, png.Length);
+            return ico;
+        }
+
         private sealed partial class RelayCommand(Action execute) : System.Windows.Input.ICommand
         {
 #pragma warning disable CS0067 // 'CanExecuteChanged' is never used — relay never changes.