How to stop preview window flickering when typing in custom text widget

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I've added a custom text widget and all works well except each key stroke causes the preview window to flicker as if reloading. It doesn't happen with the core text widget so I am assuming there is an instruction I am missing. I've read through the developer pages but nothing seems to specifically address this issue.

See video of the issue https://www.youtube.com/watch?v=LH0elBgcszU