Fluent Theme: Expander is slow to collapse in .NET 9.0.1

[bent-rasmussen]

Description

My company is preparing a preview version of our app using the new fluent theme but we were surprised to find this regression in 9.0.1 (the slowness was introduced after upgrading from SDK 9.0.101 to SDK 9.0.102) - nothing else changed:

Reproduction Steps

I have not been able to create a minimal repro yet.

Expected behavior

Fast collapse.

Actual behavior

Slow collapse.

Regression?

• It was fast in SDK 9.0.101.
• It is now slow in SDK 9.0.102.

Known Workarounds

Use prior version of .NET.

Impact

Poor UX.

Configuration

Microsoft Windows 11 Pro
Windows 10.0.26100 Build 26100
Surface Pro 8
.NET 9.0.1 (SDK 9.0.102)

Other information

No response

[dipeshmsft]

@bent-rasmussen I will take a look at this, I pushed a fix in Expander as it was causing an issue during theme change, but I didn't face this issue.

[bent-rasmussen]

Thanks. So far I can only say that we use a Frame/Page navigation setup, so that might be affecting the issue, I'm not sure.

[dipeshmsft]

This shouln't be the issue as such as the whole WPF Gallery app is based on that setup, but I will take a look at it.

[Theo-bos]

I Found the Problem, thought I could share it here.
The Problem lies in the Expander Style, it happens only on Expander with Direction = Down.

<Style x:Key="DefaultExpanderStyle" TargetType="{Type Expander}">
<ControlTemplate.Triggers>

                      <MultiTrigger.Conditions>
                                <Condition Property="Expander.IsExpanded" Value="True"/>
                                <Condition Property="Expander.ExpandDirection" Value="Down"/>
                      </MultiTrigger.Conditions>

<TriggerBase.ExitActions>
This Keytime of 5 Seconds the produces this delay
<!--<DiscreteObjectKeyFrame KeyTime="0:0:5" Value="{x:Static Visibility.Collapsed}"/>-->
Should be like the other ExitActions
<DiscreteObjectKeyFrame KeyTime="0:0:0.2" Value="{x:Static Visibility.Collapsed}"/>

[dipeshmsft]

Thanks @Theo-bos for investigating the issue, if you would like to you can go ahead and raise a PR for this issue.
Meanwhile, @bent-rasmussen, I guess you can workaround it by creating a custom style from DefaultExpanderStyle and using it as the default to continue the development of your application.