Correctly query the primary button in a form (#32438)

Correctly query the primary button in a form (#32438)

The "primary button" is used at many places, but sometimes they might
conflict (due to button switch, hidden panel, dropdown menu, etc).

Sometimes we could add a special CSS class for the buttons, but
sometimes not (see the comment of QuickSubmit)

This PR introduces `querySingleVisibleElem` to help to get the correct
primary button (the only visible one), and prevent from querying the
wrong buttons.

Fix #32437

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Co-authored-by: silverwind <me@silverwind.io>