When you set the Window's Visible attribute to true and the popup renders, the Blazor framework will fire the OnAfterRenderAsync event.
The OnAfterRenderAsync event fires a lot of times during the page life cycle. You will need a boolean flag to detect when to focus the textbox (e.g. FocusFlag ). Set this flag to true when you set the Window Visible attribute to true.
The following snippet demonstrates the above instructions. I also created this REPL example that you can use as reference.
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