ThemeVariant property definitions to ThemeVariantScope + fix DataGrid theme variant switch

[maxkatz6]

What does the pull request do?

Fixes: #10292 - Popup wasn't properly inheriting theme variant from the placement target
Fixes: #10201 - Workaround for #10345, default value of actual theme now is "null" instead of "Light" (correct value is inherited)
Fixes: #9410 - Fluent DataGrid theme wasn't using theme dictionaries
Fixes: #10319 - Some Simple control themes that were ported from Fluent had the same problem as DataGrid, but instead I simplified them to be in-line with other simple control themes.
Fixes: #10309 - Invalid property name registration was breaking bindings.
Also seem to fix #10110

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0029386-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0029389-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[robloo]

I never did get through the original PR. Just noticed this though:

/// <summary>
/// Specifies a UI theme variant that should be used for the 
/// </summary>
[TypeConverter(typeof(ThemeVariantTypeConverter))]
public sealed record ThemeVariant

1. Missing comments. Just was never completed.
2. Is record really the best to use here?
   • I know it had many uses now but it was originally designed to represent a row in a database table and give boilerplate implementations for equals/etc based on values. It doesn't look quite right in this context. record class would look a bit better but I suppose that's preference.
   • You don't actually take advantage of any record features from what I see. You re-implemented Equals and GetHashCode anyway.
   • You DID NOT add new implementations for the equality operators... so == (implemented by the compiler) will be different than .Equals() which is bad practice.

Anyway, just added this here since you are making some fixes. I can delete and create a new issue if you want.

[maxkatz6]

Missing comments. Just was never completed.

Thanks, will fix that later.

record class would look a bit better but I suppose that's preference.

"record class" and "record" is the same thing. Unlikely "record struct" which is different.

You DID NOT add new implementations for the equality operators

No, record will use user defined Equals method. I did double check this.

You don't actually take advantage of any record features from what I see

Tbh, initially when I wrote this class, there was more benefits of using record. But then I changed much.
Still there are some benefits while no disadvantages.
See sharplab output for this type. You can replace "record" with "class" there and compare.
Differences:

• record automatically implements IEquatable
• record automatically defienes == and != operators
• record automatically overrides Equals(object) method and calls user defined Equals(ThemeVariant)
• record defines Clone for variant with {} syntax, which is rather useless as there are no publicly settable properties
  It also has defined PrintMembers method, which seems to be leftover from original generated ToString, which is replaced by user-written one. But since it's a private trimmable method, that's not a problem.

[maxkatz6]

@amwx I haven't checked what's wrong again with #10110, but by any chance could it be the same as #10201? And possibly fixed in this PR.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0030054-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[amwx]

@amwx I haven't checked what's wrong again with #10110, but by any chance could it be the same as #10201? And possibly fixed in this PR.

Just tested with this branch and HighContrast theme seems to be working correctly now (both on load and switching)

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0030086-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0030339-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]