Proposal: Request more RichEditBox capabilities to support a complex text editor

Goal: To develop a complete text editor application（like "word" or "note"）

In wpf, we are used to use RichTextBox with FlowDocument for height customisation.

But in uwp and winui3, it is split into RichEditBox and RichTextBlock.

RichTextBlock can display full custom content, but can't be edited.
RichEditBox can be edited, but can't display full custom content. (or hard)

"Full customisation" = "Regular text style" + InlineUIContainer +...

RichEditBox richEditBox = new RichEditBox();
richEditBox.Document.Selection.InsertImage(); //only image

RichTextBlock richTextBlock = new RichTextBlock();
Paragraph paragraph = new Paragraph();
InlineUIContainer inlineUIContainer = new InlineUIContainer();
paragraph.Inlines.Add(inlineUIContainer);
richTextBlock.Blocks.Add(paragraph);

In addition to this, wpf's flow document supports full xaml writing. And winui3 doesn't support it.

Start by reviewing the RichEditBox and RichTextBlock APIs and the sample code in the issue, focusing on the requested custom content and XAML-writing capabilities. Done requires defining a concrete, implementable scope for the proposed RichEditBox improvements rather than only comparing WPF, UWP, and WinUI 3 behavior.