[DarkMode] Transient child forms are not visually observable in Dark Mode while clearly visible in classic mode

[Olina-Zhang]

.NET version

.NET11 SDK build: 11.0.100-rc.1.26413.103 + private dlls built from main branch of Winforms repo

Did it work in .NET Framework?

Not tested/verified

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

Yes, it's a preview regression, regressed from .NET 11 Preview7 milestone

Issue description

When repeatedly creating and immediately closing parent and child forms, transient window flashing is clearly visible in Classic Mode but is barely or not observable at all in Dark Mode.

The same code path is executed in both modes. The forms are successfully created, shown, and closed. However, the visual presentation behavior differs significantly between Classic Mode and Dark Mode.

This makes Dark Mode behavior inconsistent with Classic Mode and may indicate an issue in WinForms Dark Mode rendering or window presentation timing.

Also repro in FormOwnerTest in https://github.com/dotnet/winforms/tree/main/src/test/integration/WinformsControlsTest app under DarkMode

DarkMode:

ClassicMode:

Steps to reproduce

WinFormsApp119.zip

In app's Program.cs file, comment and uncomment the following code line to run and observe:
Application.SetColorMode(SystemColorMode.Dark);

[KlausLoeffelmann]

That is not a bug, that is a feature.
We have a new API in .NET 11 called

Application.SetDefaultFormRevealMode(FormRevealMode.Classic|Deferred|Inherits)

With the API on Application, we can set this globally for the whole app.
Now. There is a subtle "specialty" here, which is documented:

Since DarkMode was new only in .NET 10, we cannot regress any substantial behavior for DarkMode. For that reason, we decided that this is the default setting for the FormRevealMode:

• ClassicMode: FormRevealMode.Classic
• DarkMode: FormRevealMode.Deferred

Now. What's the FormRevealMode for? Basically, it's meanwhile an A11Y feature to avoid white flashing of appearing (revealing) Windows/Forms in DarkMode.

Every Window which gets created in Windows has from the beginning a white background, and that cannot be changed. So, even if you set the background to a darker color at the earliest possible time, you will see the white background always for a split second. Except: If you cloak the Window. And that is exactly, what we're doing here. A LOT of Apps have issues with this. This video shows a few of them in slow motion - I think that makes it pretty clear.

FormRevealMode.mp4

A question I have, anyway:
What was this unit test actually testing?

[Olina-Zhang]

@KlausLoeffelmann Thanks for the explanation. I did some additional testing and wanted to share the results.

Test 1 - Dark Mode + FormRevealMode.Classic

I explicitly set:
Application.SetDefaultFormRevealMode(FormRevealMode.Classic);

Under this configuration, I can observe the child forms being repeatedly opened and closed during the loop. The behavior is much closer to what I see under Classic Theme and seems consistent with the documented difference between Classic and Deferred reveal modes.

Test 2 - Dark Mode + FormRevealMode.Deferred

I switched back to:

Application.SetDefaultFormRevealMode(FormRevealMode.Deferred);

and modified the sample to keep the child form alive longer:

public Form CreateChildFormWithMemory()
{
_array = new byte[1024 * 1024];
Array.Clear(_array, 0, _array.Length);
Form child = new();

Thread.Sleep(1000);

child.Show(this);

return child;
}

Even after introducing the delay, I still cannot see the child form appear on the desktop.
However, the child form does appear in the taskbar preview/thumbnails, which suggests that the window has been created successfully and is known to DWM.

This is the part I am trying to understand better. In Deferred mode, is it expected that an owned form shown via:
child.Show(this);
can exist and appear in taskbar previews while remaining invisible on the desktop?

Would you mind taking a look using the sample application or our FormOwnerTest in https://github.com/dotnet/winforms/tree/main/src/test/integration/WinformsControlsTest ? I'd like to better understand whether this is expected behavior for owner/owned form scenarios in Deferred mode or whether there is something else involved.

[Olina-Zhang]

What was this unit test actually testing?

FormOwnerTest test is to verify that Show(owner) does not cause a memory leak. It repeatedly creates parent and child forms, allocates memory on the parent, closes the forms, forces garbage collection, and compares memory usage before and after GC.

The Dark Mode / FormRevealMode behavior was something I noticed while running the test