[Regression] Form title bars are the wrong color (app with its own custom dark theming)

[rickbrew]

.NET version

.NET 9.0.0-rc.1.24421.1 (downloaded at https://github.com/dotnet/sdk/blob/main/documentation/package-table.md)

Did it work in .NET Framework?

Yes

Did it work in any of the earlier releases of .NET Core or .NET 5+?

Yes, it works fine in .NET 8. This only happens when I change the target framework to 9.

Issue description

I compiled my app (Paint.NET) for .NET 9-rc1. From my understanding this is the first build with the experimental dark mode functionality. Paint.NET has its own extensive Dark Mode theming with lots of custom painting, setting the right DWM attributes, and even sending a few not-really-documented-but-not-really-undocumented window messages to coerce certain controls to run in Dark Mode. For now I will not be using any of that, I am simply working on getting Paint.NET to boot up correctly in .NET 9 so that I can ship my own beta to the public and get more testing and validation.

This is how PDN should look when booting it up. This is running on .NET 8:

If I make one change, which is to target .NET 9 with this .NET 9-rc1 build, it looks like this:

Notice how the 4 floating window now have white title bars. This affects all child dialogs. The main window is not affected because I'm doing some very intensive (aka crazy) custom non-client painting in there.

I tracked this down to code in Control.SetVisibleCore(). It is calling a PrepareDarkMode method which unconditionally calls DwmSetWindowAttribute to set the DWMWA_USE_IMMERSIVE_DARK_MODE attribute -- even if none of the experimental dark theme flags are enabled!

winforms/src/System.Windows.Forms/src/System/Windows/Forms/Control.cs

Line 10572 in 062b0fe

PrepareDarkMode(HWND, Application.IsDarkModeEnabled);

I can almost work around this by overriding SetVisibleCore in the derived class, like so:

    protected override void SetVisibleCore(bool value)
    {
        base.SetVisibleCore(value);

        // This is my method where I handle all sorts of theming stuff, including calling 
        // `DwmSetWindowAttribute` to set the DWMWA_USE_IMMERSIVE_DARK_MODE 
        // attribute to the appropriate value based on my app's configuration
        RefreshGlass(false); 
    }

However, this causes a different problem: when a Form is first shown, you can see the title bar start out as white and then animate towards the correct black color. This does not happen on .NET 8. This is clearly happening because the DWMWA_USE_IMMERSIVE_DARK_MODE attribute is being set twice:

Desktop.2024.08.29.-.17.15.03.02.mp4

Paint.NET already manages this attribute, so I need WinForms to not manage it at all. I haven't enabled the experimental Dark Theme stuff, so I would expect literally no effect on any of my own custom theming, much of which will -- at least for now -- overlap with the experimental .NET dark theme stuff.

cc @KlausLoeffelmann

Steps to reproduce

I can provide access to a Paint.NET build and/or repository for reproducing this, if necessary.

[rickbrew]

I'm not sure why this was tagged as help-wanted 🤔 This is @KlausLoeffelmann 's area, right? If he's on vacation or something then I can try to take a stab at it, but theming tends to be an area with a lot of delicate dependencies and I think Klaus would be able to figure it out a lot faster and a lot more safely than me.

[kirsan31]

@rickbrew may be confusion here is due to topic subject? I think we need something like "[regression] .... "

[rickbrew]

I guess my point is that this isn't something where I'm trying to use the new Dark Mode experimental features. If that were the case, I'd expect it to have some bugs that could be fixed later.

This is a situation where I'm explicitly not using it, but it's still breaking my app in a way that prevents me from migrating to .NET 9.

[kirsan31]

@KlausLoeffelmann @JeremyKuhne @elachlan @merriemcgaw @Tanya-Solyanik
Someone point this issue to the right (regression) direction pls🙏

[JeremyKuhne]

Sorry this got tagged incorrectly. Looking at it now.