How to create a text editor that shows auto-completion suggestions in the popup (similar to entering email address in Outlook)

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I have strange behavior when i used it in combination with Mask. I set MaskType = RegEx and I have an EditMask like this: (\p{Lu}|\d{1}).* meaning I want the first character upper. On execution when i type the first character into TextEditPopUp, the cursor go back to first position selecting the character. It’s like when i press the INS button on keyboard. I tested and it happens only when Mask is used.

Do you have an example how to do this in WPF?
I tried it with the xpf LookUpEdit control by setting StyleSettings to MultiSelectLookUpEditStyleSettings, TextIsEditable = "True" and IncrementalFiltering="True". For the first value it works fine, but after entering the Separator ";" and start entering the second value auto-completion does no longer work.