[QuickAccent] Update Topmost logic to attempt to fix hybrid graphics issues (#41044)

An attempt to fix a Quick Accent issue affecting laptops with 'Optimus'
hybrid graphics modes, where the utility locks the machine into discrete
graphics mode permanently.

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

This PR changes the Topmost behaviour for Quick Accent from always true
to only being true when the selection window is displayed.

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## PR Checklist

- [x] Closes: #34849 (NB: requires testing on laptop with hybrid
graphics)
- [ ] **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

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

Topmost was set to `true` for the main application window on start,
which persisted for the lifetime of the application. This PR change
removes that, and instead dynamically toggles Topmost for the selection
window as it is activated/deactivated. The assumption is that the
FluentWindow-derived window is retaining graphics resources and
presenting as an active GPU consumer because of the main application
window's Topmost status, even if the selection window is hidden.

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

I've confirmed with manual tests that the existing QuickAccent
functionality appears to function identically with this change. However,
I do not own a laptop with a hybrid graphics capability, so am unable to
test whether this fixes the underlying problem. I will keep my fingers
crossed though 🤞😊

---------

Co-authored-by: Gleb Khmyznikov <gleb.khmyznikov@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/modules/poweraccent/PowerAccent.UI/Selector.xaml.cs

@@ -44,7 +44,6 @@ public partial class Selector : FluentWindow, IDisposable, INotifyPropertyChange
         Wpf.Ui.Appearance.SystemThemeWatcher.Watch(this);
 
         Application.Current.MainWindow.ShowActivated = false;
-        Application.Current.MainWindow.Topmost = true;
     }
 
     protected override void OnSourceInitialized(EventArgs e)
@@ -64,6 +63,9 @@ public partial class Selector : FluentWindow, IDisposable, INotifyPropertyChange
 
     private void PowerAccent_OnChangeDisplay(bool isActive, string[] chars)
     {
+        // Topmost is conditionally set here to address hybrid graphics issues on laptops.
+        this.Topmost = isActive;
+
         CharacterNameVisibility = _powerAccent.ShowUnicodeDescription ? Visibility.Visible : Visibility.Collapsed;
 
         if (isActive)