CmdPal: Add drag & drop support (#44165)

## Summary of the Pull Request

This PR adds basic drag-and-drop support for items in list and grid
views.

It introduces two new properties on `ListItem`, backed by
`IExtendedAttributesProvider`: `DataPackage` and `DataPackageView`.
These properties are mutually exclusive.
`DataPackage` serves as a convenience property allowing the item to
retain the underlying object without risk of losing it. Across the
extension boundary, only the immutable `DataPackageView` snapshot is
transferred. When `DataPackage` is set, `DataPackageView` is derived
from it.

This PR includes initial concrete drag-and-drop implementations for:
- File Indexer  
- Clipboard History  

**Todo / Missing pieces** 
- [x] Extend `DataPackage` support to top-level command items, enabling
scenarios such as index fallback ~
- [x] Provide automatic drag-and-drop for unconfigured list items (e.g.,
copying title and subtitle as text)
- [x] Keep CmdPal open
- [ ] ~Clipboard commands (since we have the DataPackage...)~
- [ ] ~Improve logging~

## Pictures? Moving ones!


https://github.com/user-attachments/assets/13eb9a71-e760-43ea-8c2d-cd41cf377905




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## PR Checklist

- [x] Closes: #38289 
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/MainWindow.xaml.cs

@@ -52,6 +52,8 @@ public sealed partial class MainWindow : WindowEx,
     IRecipient<ShowWindowMessage>,
     IRecipient<HideWindowMessage>,
     IRecipient<QuitMessage>,
+    IRecipient<DragStartedMessage>,
+    IRecipient<DragCompletedMessage>,
     IDisposable
 {
     private const int DefaultWidth = 800;
@@ -79,6 +81,8 @@ public sealed partial class MainWindow : WindowEx,
 
     private WindowPosition _currentWindowPosition = new();
 
+    private bool _preventHideWhenDeactivated;
+
     private MainWindowViewModel ViewModel { get; }
 
     public MainWindow()
@@ -119,6 +123,8 @@ public sealed partial class MainWindow : WindowEx,
         WeakReferenceMessenger.Default.Register<QuitMessage>(this);
         WeakReferenceMessenger.Default.Register<ShowWindowMessage>(this);
         WeakReferenceMessenger.Default.Register<HideWindowMessage>(this);
+        WeakReferenceMessenger.Default.Register<DragStartedMessage>(this);
+        WeakReferenceMessenger.Default.Register<DragCompletedMessage>(this);
 
         // Hide our titlebar.
         // We need to both ExtendsContentIntoTitleBar, then set the height to Collapsed
@@ -751,6 +757,12 @@ public sealed partial class MainWindow : WindowEx,
                 return;
             }
 
+            // We're doing something that requires us to lose focus, but we don't want to hide the window
+            if (_preventHideWhenDeactivated)
+            {
+                return;
+            }
+
             // This will DWM cloak our window:
             HideWindow();
 
@@ -1027,4 +1039,44 @@ public sealed partial class MainWindow : WindowEx,
         _windowThemeSynchronizer.Dispose();
         DisposeAcrylic();
     }
+
+    public void Receive(DragStartedMessage message)
+    {
+        _preventHideWhenDeactivated = true;
+    }
+
+    public void Receive(DragCompletedMessage message)
+    {
+        _preventHideWhenDeactivated = false;
+        Task.Delay(200).ContinueWith(_ =>
+        {
+            DispatcherQueue.TryEnqueue(StealForeground);
+        });
+    }
+
+    private unsafe void StealForeground()
+    {
+        var foregroundWindow = PInvoke.GetForegroundWindow();
+        if (foregroundWindow == _hwnd)
+        {
+            return;
+        }
+
+        // This is bad, evil, and I'll have to forgo today's dinner dessert to punish myself
+        // for  writing this. But there's no way to make this work without it.
+        // If the window is not reactivated, the UX breaks down: a deactivated window has to
+        // be activated and then deactivated again to hide.
+        var currentThreadId = PInvoke.GetCurrentThreadId();
+        var foregroundThreadId = PInvoke.GetWindowThreadProcessId(foregroundWindow, null);
+        if (foregroundThreadId != currentThreadId)
+        {
+            PInvoke.AttachThreadInput(currentThreadId, foregroundThreadId, true);
+            PInvoke.SetForegroundWindow(_hwnd);
+            PInvoke.AttachThreadInput(currentThreadId, foregroundThreadId, false);
+        }
+        else
+        {
+            PInvoke.SetForegroundWindow(_hwnd);
+        }
+    }
 }