[UWP] Fix MultiWindow crash using default brushes

[jsuarezruiz]

Description of Change

Fix MultiWindow crash using default brushes on UWP.

Issues Resolved

• fixes [Bug] Predefined Brushes fail in a multi-window app #14210

API Changes

None

Platforms Affected

• UWP

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Launch Core Gallery on Windows. Launch several times (more than one time) a new Window (tap the "Open Secondary Window" Button). Without exceptions, the test has passed.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[KPixel]

Can you please explain how this approach works?

It seems to take the same Brush instance, and update its Parent every time the brush is used somewhere. This change triggers a PropertyChanged event that is propagated to the other (past) parents (which are in different still-opened windows) from the "Device Dispatcher thread"...
So, in my app, I still get the exception RPC_E_WRONG_THREAD.

By the way, should this fix be applied to MAUI?
It probably has the same issue:

https://github.com/dotnet/maui/blob/ac11efee51876880e4029f055fdd3976f6a09514/src/Controls/src/Core/VisualElement.cs#L224

[KPixel]

For context, the solution that works for me is to change all default brushes.

// From:
public static readonly SolidColorBrush AliceBlue = new SolidColorBrush(Color.AliceBlue);

// To:
[System.ThreadStatic]
private static SolidColorBrush _aliceBlue;
public static SolidColorBrush AliceBlue => _aliceBlue ??= new SolidColorBrush(Color.AliceBlue);

I automated that change with a macro.

Edit: The brush's Parent still gets changed at every use, but it only applies to the same Dispatcher/thread, so no exceptions.

A small upside is that these 141 brushes get instantiated on-demand now, instead of all at once.