Use dynamic resources in Expander button theme.

[grokys]

What does the pull request do?

I'm not sure why, but the fluent Expander theme uses a mix of static and dynamic resources with seemingly no reason to differentiate them. It's not so much a problem for the resources in the default values for Expander as these can be overridden with local values, but for the toggle button and trigger styles, it means there's no way to customize these values.

Converted all of the StaticResources to DynamicResources.

Pinging @robloo who wrote this theme to check whether this change makes sense, or whether the static resources were there for a reason.

[robloo]

@grokys The Fluent Theme for Expander started as a copy paste of upstream which also uses static resources several places.

https://github.com/microsoft/microsoft-ui-xaml/blob/winui2/main/dev/Expander/Expander.xaml

Over time we've converted some of then to dynamic as needed, above what had to change during the initial port.

There are a few reasons:

1. Upstream uses static resources several places for performance reasons. The general thinking as I understand it is size-related properties (thickness, height, etc.) should be static and not changed at runtime. Brushes controlling color are full lightweight styling resources and can be changed at runtime. Or, any brushes that need to change on theme switch aren't static, everything else is a StaticResource.

2. The bigger reason is WinUI doesn't even have DynamicResource it has StaticResoirce and ThemeResource. TheneResource is only re-evaluated on theme changes. This was a major decision for performance early in WinRT/UWP.

3. For the port basically all ThemeResource usage was changed to DynamicResource and all others were kept the same.

So... the style was following upstream WinUI is basically the explanation here along with specifics noted above.

If you aren't concerned with the overhead of using DynamicResource for everything this PR is fine. Keep in mind Fluent theme basically moves everything possible out to a lightweight styling resource so there are a lot of them. Again, some things like thickness really don't change all that much.

Finally, I doubt Expander is the only theme ported from WinUI just changing ThemeResource to DynamicResource. I bet StaticResource comes up like this in other control themes.

[avaloniaui-bot]

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[grokys]

The reason this came about was that I wanted to create a borderless Expander with no padding in the header area. Because ExpanderHeaderPadding is a static resource, I couldn't do that.

For the default values of Expander it doesn't really matter if they're static or dynamic because you can override them as local values, but for the toggle button in the header, they really need to be.