Hello,
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Yes, currently, there is no way to customize the TextControlCommandBarContextFlyout which is used in RichEditBox as ContextFlyout. And another thing is that the Proofing function is implemented by the system and you could not customize it as well. If you want to use a custom ContextFlyout to replace the default TextControlCommandBarContextFlyout, then you will not be able to use the Proofing function.
Thank you.
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