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/ExtViews/ListPage.xaml

@@ -439,9 +439,12 @@
                         <ListView
                             x:Name="ItemsList"
                             Padding="0,2,0,0"
+                            CanDragItems="True"
                             ContextCanceled="Items_OnContextCanceled"
                             ContextRequested="Items_OnContextRequested"
                             DoubleTapped="Items_DoubleTapped"
+                            DragItemsCompleted="Items_DragItemsCompleted"
+                            DragItemsStarting="Items_DragItemsStarting"
                             IsDoubleTapEnabled="True"
                             IsItemClickEnabled="True"
                             ItemClick="Items_ItemClick"
@@ -458,9 +461,12 @@
                         <GridView
                             x:Name="ItemsGrid"
                             Padding="16,0"
+                            CanDragItems="True"
                             ContextCanceled="Items_OnContextCanceled"
                             ContextRequested="Items_OnContextRequested"
                             DoubleTapped="Items_DoubleTapped"
+                            DragItemsCompleted="Items_DragItemsCompleted"
+                            DragItemsStarting="Items_DragItemsStarting"
                             IsDoubleTapEnabled="True"
                             IsItemClickEnabled="True"
                             ItemClick="Items_ItemClick"

src/modules/cmdpal/Microsoft.CmdPal.UI/ExtViews/ListPage.xaml.cs

@@ -18,6 +18,7 @@ using Microsoft.UI.Xaml.Controls.Primitives;
 using Microsoft.UI.Xaml.Input;
 using Microsoft.UI.Xaml.Media;
 using Microsoft.UI.Xaml.Navigation;
+using Windows.ApplicationModel.DataTransfer;
 using Windows.Foundation;
 using Windows.System;
 
@@ -891,6 +892,89 @@ public sealed partial class ListPage : Page,
         ItemView.SelectedIndex = newIndex;
     }
 
+    private void Items_DragItemsStarting(object sender, DragItemsStartingEventArgs e)
+    {
+        try
+        {
+            if (e.Items.FirstOrDefault() is not ListItemViewModel item || item.DataPackage is null)
+            {
+                e.Cancel = true;
+                return;
+            }
+
+            // copy properties
+            foreach (var (key, value) in item.DataPackage.Properties)
+            {
+                try
+                {
+                    e.Data.Properties[key] = value;
+                }
+                catch (Exception)
+                {
+                    // noop - skip any properties that fail
+                }
+            }
+
+            // setup e.Data formats as deferred renderers to read from the item's DataPackage
+            foreach (var format in item.DataPackage.AvailableFormats)
+            {
+                try
+                {
+                    e.Data.SetDataProvider(format, request => DelayRenderer(request, item, format));
+                }
+                catch (Exception)
+                {
+                    // noop - skip any formats that fail
+                }
+            }
+
+            WeakReferenceMessenger.Default.Send(new DragStartedMessage());
+        }
+        catch (Exception ex)
+        {
+            WeakReferenceMessenger.Default.Send(new DragCompletedMessage());
+            Logger.LogError("Failed to start dragging an item", ex);
+        }
+    }
+
+    private static void DelayRenderer(DataProviderRequest request, ListItemViewModel item, string format)
+    {
+        var deferral = request.GetDeferral();
+        try
+        {
+            item.DataPackage?.GetDataAsync(format)
+                .AsTask()
+                .ContinueWith(dataTask =>
+                {
+                    try
+                    {
+                        if (dataTask.IsCompletedSuccessfully)
+                        {
+                            request.SetData(dataTask.Result);
+                        }
+                        else if (dataTask.IsFaulted && dataTask.Exception is not null)
+                        {
+                            Logger.LogError($"Failed to get data for format '{format}' during drag-and-drop", dataTask.Exception);
+                        }
+                    }
+                    finally
+                    {
+                        deferral.Complete();
+                    }
+                });
+        }
+        catch (Exception ex)
+        {
+            Logger.LogError($"Failed to set data for format '{format}' during drag-and-drop", ex);
+            deferral.Complete();
+        }
+    }
+
+    private void Items_DragItemsCompleted(ListViewBase sender, DragItemsCompletedEventArgs args)
+    {
+        WeakReferenceMessenger.Default.Send(new DragCompletedMessage());
+    }
+
     /// <summary>
     ///  Code stealed from <see cref="Controls.ContextMenu.NavigateDown"/>
     /// </summary>