Hi Vernon Marsden,
What version of your Visual Studio is and what target framework your project is?
How did you create the textbox control? Please provide more details to reproduce the problem.
And you can try the following suggestion.
You can add another textBox from ToolBox to Form1 and check if it also doesn't show on the DesignForm.
Please try to restart or update your Visual Studio.
Best Regards,
Daniel Zhang
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