Support for StartAllBack multitaskbar notification area

StartAllBack provides support for per-taskbar notification area.
Shell APIs would return active (last clicked) taskbar.

- Get active taskbar rect via ABM_GETTASKBARPOS with special flag; use that rect to get monitor instead of primary tray monitor
- Get taskbar DPI via proper API which would account for active / auto-hidden taskbar
- Readjust popup window position by repeating calculation with new DPI to account for DPI changes

File-New-Project#1611

Author: Simplestas

EarTrumpet/Interop/Helpers/WindowsTaskbar.cs

@@ -34,7 +34,7 @@ public static uint Dpi
         {
             unsafe
             {
-                return PInvoke.GetDpiForWindow(new HWND(GetHwnd()));
+                return PInvoke.GetDpiForShellUIComponent(SHELL_UI_COMPONENT.SHELL_UI_COMPONENT_NOTIFICATIONAREA);
             }
         }
     }
@@ -53,7 +53,7 @@ public static State Current
                 var appBarData = new APPBARDATA
                 {
                     cbSize = (uint)Marshal.SizeOf(typeof(APPBARDATA)),
-                    hWnd = new HWND(hwnd)
+                    lParam = 0x42 // magic to get active taskbar rect with StartAllBack
                 };
 
                 // SHAppBarMessage: Understands Taskbar auto-hide
@@ -62,6 +62,7 @@ public static State Current
                 {
                     state.Size = appBarData.rc;
                     state.Location = (Position)appBarData.uEdge;
+                    state.ContainingScreen = Screen.FromRectangle((System.Drawing.Rectangle)appBarData.rc);
                 }
                 else
                 {

EarTrumpet/NativeMethods.txt

@@ -33,6 +33,7 @@ GET_GUI_RESOURCES_FLAGS
 GetApplicationUserModelId
 GetClassName
 GetCurrentProcess
+GetDpiForShellUIComponent
 GetDpiForSystem
 GetDpiForWindow
 GetForegroundWindow

EarTrumpet/UI/Views/FlyoutWindow.xaml.cs

@@ -49,7 +49,11 @@ private void OnStateChanged(object sender, object e)
 
                 Show();
                 EnableAcrylicIfApplicable(taskbar);
+                var prevDpi = this.DpiX();
                 PositionWindowRelativeToTaskbar(taskbar);
+                // Account for DPI changing due to SetWindowPos
+                if (this.DpiX() != prevDpi)
+                    PositionWindowRelativeToTaskbar(taskbar);
 
                 // Focus the first device if available.
                 DevicesList.FindVisualChild<DeviceView>()?.FocusAndRemoveFocusVisual();