Always on top: Add transparent support for on topped window (#44815)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Transparency support (best-effort)
> Not every window can be made transparent. Transparency is applied on a
best-effort basis and depends on how the target app/window is built and
rendered.

## When it may not work
* Windows with special rendering pipelines (e.g., certain
hardware-accelerated / compositor-managed surfaces).
* Some tool/popup/owned windows where the foreground window isn’t the
actual surface being drawn.

## How it works (high-level)
* Resolve the best target window (preferring the top-level/root window
over transient children).
* Apply Windows’ standard layered-window alpha mechanism (per-window
opacity) to adjust transparency.
* When unpinned, Restore the original opacity/state when possible.

If transparency doesn’t change, it means the window doesn’t support this
mechanism in its current configuration.
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## PR Checklist

- [X] Closes: 
#43278 
#42929
#28773

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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
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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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed


https://github.com/user-attachments/assets/c97a87f2-3126-4e19-990f-8c684dbeb631

<img width="1119" height="426" alt="image"
src="https://github.com/user-attachments/assets/547671ee-81d3-4c94-8199-bf0c4b1b7760"
/>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/settings-ui/Settings.UI/ViewModels/AlwaysOnTopViewModel.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text.Json;
 using global::PowerToys.GPOWrapper;
@@ -133,6 +134,10 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
                     Settings.Properties.Hotkey.Value = _hotkey;
                     NotifyPropertyChanged();
 
+                    // Also notify that transparency keys have changed
+                    OnPropertyChanged(nameof(IncreaseOpacityKeysList));
+                    OnPropertyChanged(nameof(DecreaseOpacityKeysList));
+
                     // Using InvariantCulture as this is an IPC message
                     SendConfigMSG(
                         string.Format(
@@ -289,6 +294,62 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
             }
         }
 
+        /// <summary>
+        /// Gets the keys list for increasing window opacity (modifier keys + "+").
+        /// </summary>
+        public List<object> IncreaseOpacityKeysList
+        {
+            get
+            {
+                var keys = GetModifierKeysList();
+                keys.Add("+");
+                return keys;
+            }
+        }
+
+        /// <summary>
+        /// Gets the keys list for decreasing window opacity (modifier keys + "-").
+        /// </summary>
+        public List<object> DecreaseOpacityKeysList
+        {
+            get
+            {
+                var keys = GetModifierKeysList();
+                keys.Add("-");
+                return keys;
+            }
+        }
+
+        /// <summary>
+        /// Gets only the modifier keys from the current hotkey setting.
+        /// </summary>
+        private List<object> GetModifierKeysList()
+        {
+            var modifierKeys = new List<object>();
+
+            if (_hotkey.Win)
+            {
+                modifierKeys.Add(92); // The Windows key
+            }
+
+            if (_hotkey.Ctrl)
+            {
+                modifierKeys.Add("Ctrl");
+            }
+
+            if (_hotkey.Alt)
+            {
+                modifierKeys.Add("Alt");
+            }
+
+            if (_hotkey.Shift)
+            {
+                modifierKeys.Add(16); // The Shift key
+            }
+
+            return modifierKeys;
+        }
+
         public void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
         {
             OnPropertyChanged(propertyName);