Stop HTML output writing to file stream

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I am using the following to force a file dialog box to open

header('Content-Disposition: attachment; filename="'.$filename.'"');
header('Content-type: text/csv');

Then open a file stream with

return $this->handle = fopen('php://output','w');

Which directs a file strean to output rather than write to a file.
Well that is how I understand it.
I then write the data then close the stream with

fclose($this->handle);

Problem is html output which I expect to display in the browser continues to write to the stream,
so HTML appears at the bottom of the CSV when opened.

What am I missing to close the file stream and output HTML?