Win32 Taskbar Hooking & Overlay Interop

LocalTelemetry embeds a high-performance GDI+ render surface (NativeWindow) directly into the native Windows taskbar process (Shell_TrayWnd).

🛠️ Win32 Taskbar Hierarchy

The taskbar overlay management is handled inside TaskbarOverlay.cs and TaskbarEmbedder.cs (src/LocalTelemetry.App/Overlay/ & Win32/).

Embedding Approaches (TaskbarEmbedder)

LocalTelemetry implements a 2-tier embedding strategy:

1. Approach A: Child-Window Reparenting (IsChildMode)

• Find Taskbar HWND: FindWindow("Shell_TrayWnd", null) locates the primary taskbar window handle.
• Find ReBar / TaskBand: FindWindowEx(trayHwnd, IntPtr.Zero, "ReBarWindow32", null) retrieves the control container and taskband (MSTaskSwWClass).
• Reparent Window (SetParent): Calls Win32 SetParent(overlayHwnd, parentHwnd) to reparent the overlay window as a native child control of the taskbar.
• Shrink TaskBand (ShrinkTaskBand): Dynamically reduces taskband width to reserve dedicated space for the overlay metrics on the taskbar.
• Apply Window Styles (SetWindowLong): Sets WS_CHILD, WS_EX_TOOLWINDOW (hides from Alt+Tab), WS_EX_NOACTIVATE, and WS_EX_LAYERED.

2. Approach B: Layered Floating Window Fallback (IsFallback)

• Used if reparenting fails due to third-party shell software or security policies.
• Restores top-level window styles (WS_POPUP, WS_EX_TOPMOST) and positions a transparent layered floating window precisely over the taskbar bounding rectangle.

DPI Awareness & Scaling

Queries GetDpiForWindow to dynamically scale GDI+ font rendering, text margins, and line heights for 4K displays and custom DPI scaling settings (125%, 150%, 200%+).

Windows Explorer Restart Recovery

When explorer.exe crashes or restarts, all child handles of Shell_TrayWnd are destroyed by Windows.

LocalTelemetry overrides the Win32 WndProc message hook to listen for the registered Win32 message TaskbarCreated:

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == _taskbarCreatedMsg)
    {
        Log.Info("TaskbarCreated message received. Re-attaching overlay...");
        ReattachOverlay();
    }
    return IntPtr.Zero;
}

This guarantees seamless recovery without user intervention.