CmdPal: Prevent PgUp/PgDown from selecting non-internactive items (#46439)

## Summary of the Pull Request

This PR prevents paging (<kbd>PgUp</kbd>/<kbd>PgDown</kbd>) in the item
list from selecting non-interactive items (such as separators or section
headers).

It adds `FindSelectableIndex` and `FindSelectableIndexForPageNavigation`
helper methods, which locate the next interactive item in the given
direction. These methods are used to guard paging navigation and to
consolidate related logic elsewhere.

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## PR Checklist

- [x] Closes: #46283 
<!-- - [ ] 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.cs

@@ -156,21 +156,7 @@ public sealed partial class ListPage : Page,
     /// </summary>
     private int GetFirstSelectableIndex()
     {
-        var items = ItemView.Items;
+        return FindSelectableIndex(0, 1);
-        if (items is null || items.Count == 0)
-        {
-            return -1;
-        }
-
-        for (var i = 0; i < items.Count; i++)
-        {
-            if (!IsSeparator(items[i]))
-            {
-                return i;
-            }
-        }
-
-        return -1;
     }
 
     [System.Diagnostics.CodeAnalysis.SuppressMessage("CodeQuality", "IDE0051:Remove unused private members", Justification = "VS is too aggressive at pruning methods bound in XAML")]
@@ -604,9 +590,44 @@ public sealed partial class ListPage : Page,
             ? Math.Min(itemCount - 1, currentIndex + Math.Max(1, itemsPerPage))
             : Math.Max(0, currentIndex - Math.Max(1, itemsPerPage));
 
+        targetIndex = FindSelectableIndexForPageNavigation(targetIndex, isPageDown);
+        if (targetIndex == -1)
+        {
+            return null;
+        }
+
         return (currentIndex, targetIndex);
     }
 
+    private int FindSelectableIndex(int startIndex, int step)
+    {
+        var items = ItemView.Items;
+        var count = items.Count;
+        if (count == 0 || step == 0)
+        {
+            return -1;
+        }
+
+        startIndex = Math.Clamp(startIndex, 0, count - 1);
+
+        for (var i = startIndex; i >= 0 && i < count; i += step)
+        {
+            if (!IsSeparator(items[i]))
+            {
+                return i;
+            }
+        }
+
+        return -1;
+    }
+
+    private int FindSelectableIndexForPageNavigation(int targetIndex, bool isPageDown)
+    {
+        var step = isPageDown ? 1 : -1;
+        var index = FindSelectableIndex(targetIndex, step);
+        return index != -1 ? index : FindSelectableIndex(targetIndex - step, -step);
+    }
+
     private static void OnViewModelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
         if (d is ListPage @this)
@@ -1045,40 +1066,25 @@ public sealed partial class ListPage : Page,
     /// </summary>
     private void NavigateUp()
     {
-        var newIndex = ItemView.SelectedIndex;
+        if (ItemView.Items.Count == 0)
-
-        if (ItemView.SelectedIndex > 0)
         {
-            newIndex--;
+            return;
-
-            while (
-                newIndex >= 0 &&
-                IsSeparator(ItemView.Items[newIndex]) &&
-                newIndex != ItemView.SelectedIndex)
-            {
-                newIndex--;
         }
 
-            if (newIndex < 0)
+        var newIndex = ItemView.SelectedIndex > 0
-            {
+            ? FindSelectableIndex(ItemView.SelectedIndex - 1, -1)
-                newIndex = ItemView.Items.Count - 1;
+            : -1;
 
-                while (
+        if (newIndex == -1)
-                    newIndex >= 0 &&
-                    IsSeparator(ItemView.Items[newIndex]) &&
-                    newIndex != ItemView.SelectedIndex)
         {
-                    newIndex--;
+            newIndex = FindSelectableIndex(ItemView.Items.Count - 1, -1);
-                }
-            }
-        }
-        else
-        {
-            newIndex = ItemView.Items.Count - 1;
         }
 
+        if (newIndex != -1)
+        {
             ItemView.SelectedIndex = newIndex;
         }
+    }
 
     private void Items_DragItemsStarting(object sender, DragItemsStartingEventArgs e)
     {
@@ -1168,51 +1174,25 @@ public sealed partial class ListPage : Page,
     /// </summary>
     private void NavigateDown()
     {
-        var newIndex = ItemView.SelectedIndex;
+        if (ItemView.Items.Count == 0)
-
-        if (ItemView.SelectedIndex == ItemView.Items.Count - 1)
-        {
-            newIndex = 0;
-            while (
-                newIndex < ItemView.Items.Count &&
-                IsSeparator(ItemView.Items[newIndex]))
-            {
-                newIndex++;
-            }
-
-            if (newIndex >= ItemView.Items.Count)
         {
             return;
         }
-        }
-        else
-        {
-            newIndex++;
 
-            while (
+        var newIndex = ItemView.SelectedIndex < ItemView.Items.Count - 1
-                newIndex < ItemView.Items.Count &&
+            ? FindSelectableIndex(ItemView.SelectedIndex + 1, 1)
-                IsSeparator(ItemView.Items[newIndex]) &&
+            : -1;
-                newIndex != ItemView.SelectedIndex)
+
+        if (newIndex == -1)
         {
-                newIndex++;
+            newIndex = FindSelectableIndex(0, 1);
         }
 
-            if (newIndex >= ItemView.Items.Count)
+        if (newIndex != -1)
         {
-                newIndex = 0;
-
-                while (
-                    newIndex < ItemView.Items.Count &&
-                    IsSeparator(ItemView.Items[newIndex]) &&
-                    newIndex != ItemView.SelectedIndex)
-                {
-                    newIndex++;
-                }
-            }
-        }
-
             ItemView.SelectedIndex = newIndex;
         }
+    }
 
     private bool IsSeparator(object? item) => item is ListItemViewModel li && !li.IsInteractive;