The specification

http://docs.oracle.com/javase/8/docs/jre/api/javaws/jnlp/javax/jnlp/ExtensionInstallerService.html#hideProgressBar--

says:

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void hideProgressBar()
Hides the progress bar. Any subsequent calls to updateProgress will force it to be visible.
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void updateProgress(int value)
Updates the progress bar.
Parameters:
value - progress bar value - should be between 0 and 100.
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Experiments with the interactive tests show that it seems like the progress bar gets shown by updateProgressBar() method after getting hidden only is the passed value is different from previously set - then the progress bar gets painted again after hiding.
This behavior contradicts the spec and causes failure of the following interactive test from JNLP TCK 8.0.20

api/javax_jnlp/ExtensionInstallerService/index.html#statusWindow

Please see the attached screenshot.