When the 'focusable' and 'focus_id' parameters are set,
`open_floating_preview` assumes that it should always move focus to an
existing floating window with the same 'focus_id'. However, there are
cases where we want to make a floating window focusable, but do not want
to focus it upon calling `open_floating_preview`. To distinguish these
cases, add a boolean parameter 'focus' that, when false, prevents
moving focus.
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