[android] fix one layer of "overdraw" by default

[jonathanpeppers]

Fixes: #18245
Context: https://developer.android.com/studio/debug/dev-options#hardware
Context: https://developer.android.com/topic/performance/rendering/inspect-gpu-rendering#debug_overdraw
Context: https://stackoverflow.com/q/6499004

There is Debug GPU Overdraw developer option in Android that can be enabled to see overdraw in the app.

• Blue: overdrawn by 1
• Green: overdrawn by 2
• Pink: overdrawn by 3
• Red: overdrawn by 4+

The .NET MAUI project template shows Blue by default, in a project with nothing on the screen.

A workaround is noted on #18245, but the best workaround would be in Platforms\Android\MainActivity.cs:

protected override void OnCreate(Bundle? savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    // Clear the Window's background
    Window?.SetBackgroundDrawable(null);
}

With this change in place, the Blue overdraw is gone in the .NET MAUI project template:

The result is .NET MAUI apps on Android should inherently use less GPU (the size of the screen), which could be noticeable on low-end devices. It could be particularly noticeable in apps that are moving into the Red range based on the developer's XAML & layout choices.

We can make this same change in .NET MAUI's default Maui.MainTheme Android theme:

<item name="android:windowBackground">@null</item>

The default project template has a XAML style that sets the background:

<Style TargetType="Page" ApplyToDerivedTypes="True">
    <Setter Property="BackgroundColor" Value="{AppThemeBinding Light={StaticResource White}, Dark={StaticResource OffBlack}}" />
</Style>

So the only case this change would be noticed (wrong color), would be if:

• Developer is using a Page with no Background set, and no Background set on the top-most layout.

• Developer removed all XAML styles or is migrating a Xamarin.Forms app

In this case, a developer could do one of:

• Apply a Background to the Page

• Extend the Maui.MainTheme style, but with a Background set

[jsuarezruiz]

Mostly all the golden tests, UITests comparing a snapshot with a reference image are failing, because:

The detail page, is renderered on top of the previous one.

See the differences:

[jonathanpeppers]

If this changes behavior in even the UI Test app, I want to discuss it before we consider this one.

It might make sense to put this in .NET 9-only, or just put it in the project template.

Customers could use this workaround for now:

protected override void OnCreate(Bundle? savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    // Clear the Window's background
    Window?.SetBackgroundDrawable(null);
}

[dainius-r]

@jonathanpeppers tried using workaround with android styles <item name="android:windowBackground">@null</item>. Changing to this, everything seems working normally except Shell modal pages won't show.

[jonathanpeppers]

except Shell modal pages won't show.

@dainius-r is there an exception? (it should appear in adb logcat output) Or can you share a screenshot?