When I want to choose gfx for focus, the button sometimes works and sometimes not

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Is there specific scenarios when it doesn't work? E.g. closed the GFX box then reopened it?

Yea, I just started to do my first focus, I clicked 5 times, on the sixth click it worked, I closed, but the submit button haven't worked, so I reloaded page and then choose GFX button didn't work at all

This shouldn't be an issue. There may be a small amount of a delay if you open->close->open as the images inside the list are still loading, but it should open again after a few seconds.