Fluent Theme in .NET 10 Plan

[dipeshmsft]

In .NET 9, we laid the foundation for integrating the Fluent theme into WPF applications. While there are still some areas to improve to make it comparable to the WinUI Fluent theme, as well as achieving parity with Aero2 and other WPF themes, we are actively working to address these gaps.

We have been maintaining a project to track all the Fluent related work items via WPF Fluent Theming in .NET 9 and have updated it again to keep in focus the timelines of our deliverables for .NET 10. Below are few of the pending work items sorted by their priority -

1. Missing Control Styles in Windows 11 Themes #8698
2. Fixing Fluent Style Issues #10393
3. Testing for Fluent Theme #8552

The aforementioned tasks are currently in progress, and we will begin submitting PRs for these items shortly. After completing these tasks, we plan to proceed with additional important tasks for Fluent (not presented in any order), which will enable more developer scenarios :

1. Enabling Fluent as Default System Theme - Next Steps ? #9283
2. Window Backdrop Support : Mica and Acrylic #8545
3. Rounded Corners #8544

Please let us know your thoughts if we have missed anything.

/cc @dotnet/dotnet-wpf-triage @robert-abeo @MichaeIDietrich @pomianowski @riverar

[lindexi]

Something about Performance: as we know, the DynamicResource may cause the performance issues. How about the feature of ThemeResource and so on? See #8599

[robert-abeo]

I'm using a Fluent theme in a large production app with the heavy merged dictionary and DynamicResource design. It works just fine and there are no performance issues noticeable to the end user. Would love to see actual data on comments like this rather than theoretical discussions.

[dipeshmsft]

@robert-abeo, I will get some metrics when I work on the task of Enabling Fluent as Default, I have some metrics for the current approach as well, but there is nothing to compare with.

However, @lindexi is right that DynamicResources and merged dictionary approach is slower as compared to loading them as default styles.

[miloush]

At some point we will have to deal with the merged dictionary situation (which is also discussed in #9283)

[dipeshmsft]

Will pick it up soon...

[dipeshmsft]

Update : .NET 10 Preview 2

• During this time frame, our main focus was on adding Missing Fluent Styles for controls in WPF. Although there haven't been a lot of activity yet, but we are planning to get this completed by .NET 10 Preview 3.
• As an other parallel, we were working on setting up the plan for Fixing Fluent styles for controls and have started with DataGrid. We will continue working on this in Preview 3.
• We are still stuck on the testing infra for Fluent styles and weren't able to work more on that in the past month. Also planning to take this up in next month.

[aquinn39]

The plan looks good! And visually, these new themes look really nice and almost identical to WinUI, so they fit in well with Windows 11. However, just had a couple of questions about some things:

• In the new themes, a new way of defining system brushes and colours was added, such that apps which use SystemColors now break in dark mode due to these colours not updating in dark mode. Is there a plan to address this issue to make it easier to migrate existing apps and so apps can easily support the older themes as well as the newer ones?

• Is there any plan to bring the new themes to older versions of .NET and the .NET Framework via a NugetPackage or update? There are still reasons to use these frameworks such as backwards compatibility and the fact that .NET Framework comes preinstalled in Windows, allowing for lightweight exes that don't need you to install any prerequisites.

• When the theme is on by default, will it be the default no matter the OS like it is now when you opt-in to them? For example, .NET 9 is compatible with Windows 10 and above, and now, in .NET 9, if you opt-in to the new themes as per here, it uses it on Windows 10 and 11. Will this be the same behaviour in .NET 10?

• Will there be any plans to update the behaviours in WPF to be more modern, in line with the new themes, such as smooth vertical scrolling support by taking into account the delta value of mouse wheel events for example? I detailed some of these old behaviours that haven't been updated to follow the rest of the operating system here: Windows 11 Themes: Update the behaviours of various controls to be more modern/inline with WinUI #9203.

[dipeshmsft]

@aquinn39 , sorry for the late response on this.

In the new themes, a new way of defining system brushes and colours was added, such that apps which use SystemColors now break in dark mode due to these colours not updating in dark mode. Is there a plan to address this issue to make it easier to migrate existing apps and so apps can easily support the older themes as well as the newer ones?

Can you share some visual examples for a better understanding. I think I understand the issue, but just wanted to be more clear on the ask. In my understanding, since the SystemColors are not updated as we move from Light to dark mode in Windows, the controls using those colors / brushes don't get the right colors, and this must be causing the issue.

---

Is there any plan to bring the new themes to older versions of .NET and the .NET Framework via a NugetPackage or update? There are still reasons to use these frameworks such as backwards compatibility and the fact that .NET Framework comes preinstalled in Windows, allowing for lightweight exes that don't need you to install any prerequisites.

We haven't thought in this direction yet. Tagging @pchaurasia14 @singhashish-wpf here.

---

When the theme is on by default, will it be the default no matter the OS like it is now when you opt-in to them? For example, .NET 9 is compatible with Windows 10 and above, and now, in .NET 9, if you opt-in to the new themes as per here, it uses it on Windows 10 and 11. Will this be the same behaviour in .NET 10?

This has yet to be decided, we will first attempt on enabling Fluent as default theme task and once we have a version of it available, we will decide on the ways how to make Fluent theme work.

---

Will there be any plans to update the behaviours in WPF to be more modern, in line with the new themes, such as smooth vertical scrolling support by taking into account the delta value of mouse wheel events for example? I detailed some of these old behaviours that haven't been updated to follow the rest of the operating system here: #9203.

This will be decided in future. Tagging @pchaurasia14 @singhashish-wpf here.

[aquinn39]

@dipeshmsft No worries, thanks for that info; that clears up a lot of questions I had.

Can you share some visual examples for a better understanding. I think I understand the issue, but just wanted to be more clear on the ask. In my understanding, since the SystemColors are not updated as we move from Light to dark mode in Windows, the controls using those colors / brushes don't get the right colors, and this must be causing the issue.
Yep, that is what I meant - so like say if I set the background of a Window to SystemBrushes.ControlBrush (a colour which is often used in a lot of dialogs (such as the About Windows dialog (winver.exe)) and is the default in Windows Forms I believe), but leave the text colours as the defaults (which normally use SystemBrushes), it will break when the Windows 11 themes are enabled and in dark mode as the text will now be using the new brushes but the background will still be using SystemBrushes, making the text very hard or impossible to read.

Old themes with dark mode enabled in Windows:

New themes with dark mode enabled in Windows:

This presents four problems:

• In apps with a large number of SystemBrushes references will have a lot of work to do to support the new themes
• It breaks support with old themes
• A developer might upgrade to .NET 10, enabling the themes, and be unaware that the dark mode has been introduced, and think that the UI works fine and not realise that enabling dark mode will break it. They could have been careful to always use SystemBrushes instead of explicitly defined colours so that the UI works with all previous themes and high contrast mode but it still will break in Windows 11 dark mode.
• Developers are used to SystemBrushes, so it's more work to understand the new system even though there's nothing inherently wrong with SystemBrushes as far as I know, so there's no real reason they should have to move away from it.

[dipeshmsft]

Update : .NET 10 Preview 3

Warning

UI breaking change in the following Fluent styles: Frame. Reverts the behavior of Aero2 in Frame control. ShowsNavigationUI is enabled by default in the new changes.

• Building on our previous announcement about Missing Fluent Styles for controls in WPF, we've implemented most of these styles. ( GroupBox, GridSplitter, Thumb, ResizeGrip, Hyperlink, Frame, NavigationWindow ). However, here are the few points that I would like to put forward :

  • DocumentViewer : Since most portion of DocumentViewer styles come from PresentationUI, we included the Aero2 style for the time being.
  • GridView : Although we have implemented the style for GridView, however the implcit loading of the style is showing some challenges. We are working on finding workarounds currently and will most probably be done by the next Preview release.
  • Frame : By default in Aero2, Frame shows the navigation menu bar, however in .NET 9, the style of Frame the default behaviour was that the MenuBar was hidden. In the current fix, we modified the value to make this similar to Aero2, so that the original default behavior is maintained. This will essentially be a UI breaking change for the applications using Fluent theme in .NET 9

• Regarding the ongoing efforts to Fixing Fluent styles for controls we have addressed few issues raised on GitHub. We also fixed the issue with LinearGradientBrush which will allow us to fix elevation-control brushes, and all the TextBox based control. We will see these PRs in .NET Preview 4.

• We have fixed the HC mode crash in WPF Gallery and will publish the new version soon. It will contain the pages for the missing style that have been added in .NET 10 Preview 3.