With the current implementation of the RadSyntaxEditor control, a TextSearchHighlightTagger needs to be registered in the TaggersRegistry for the highlighting of the find and replace dialog to work. You can register the tagger like so:
var textSearchTagger = new TextSearchHighlightTagger(this.syntaxEditor, TextSearchHighlightTagger.SearchFormatDefinition);
I have forwarded this scenario to our development team and we will consider registering this tagger out of the box for the official version of the control.
Nonetheless, I hope you find registering this tagger manually applicable for the time being. Please let me know if this is indeed the case.
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