RFR: 8370902: RichTextArea: migrate to new text APIs

[Andy Goryachev]

On Wed, 17 Dec 2025 20:38:20 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

> Updates the `RichTextArea` code to use the new text layout APIs delivered in https://bugs.openjdk.org/browse/JDK-8341670
> 
> Also:
> 
> - updates the "demo" model in the `RichTextAreaDemoApp` to show underline/strikethrough in the presence of `TextFlow` borders

thank you for reviewing!

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PR Comment: https://git.openjdk.org/jfx/pull/2012#issuecomment-3891564001