RTL Maldivian/Dhivehi (Thaana) text is not displayed correctly in WPF textboxes

[mtodkari]

• Windows version: (winver) Windows 10
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

• Security issues and bugs should be reported privately, learn more via our responsible disclosure guidelines.

  Problem description: When inputting certain right-to-left language scripts in a WPF text box with an English OS, the characters are reversed.

Issue is reproducible with certain right-to-left languages, like Dhivehi/Thaana and Uyghur/Uighur Arabic, but not others like Hebrew and Arabic.

Actual behavior: Reversed text (incorrect)

Expected behavior: Correct Text:

Minimal repro:

1. In a simple WPF application, copy the text ތާނަ from Wiki page,
2. Observe that Thaana script is shown reversed

[miloush]

When the directionality of the TextBox is set to RTL, it renders as expected (although that is not required for Arabic).

There is also a list of supported scripts by WPF, see https://docs.microsoft.com/en-us/dotnet/desktop/wpf/advanced/globalization-for-wpf?view=netframeworkdesktop-4.8#international-text.

[mtodkari]

@miloush thank you for your response. We cannot use FlowDirection="RighToLeft" because that affects the display direction in English as well. Arabic and Hebrew RTL are correctly handled by WPF and do not need any special formatting. Thaana should be handled similarly too.

https://github.com/dotnet/wpf/blob/bc968b3127f12031534e2e0674278314ec558744/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedTextBuilder.cs#L1135 Maybe Thaana just needs to be added to _IsRTL.