Drop down behavior

I see 3 types of behavior on the web for drop downs and I wanted to collect some insight.

When a user opens a drop down, I observe:

  1. the user can scroll and the drop down stays open (bootstrap)

first behavior

  1. the user can not scroll, he is locked and needs to close the drop down by clicking somewhere else (safari and IE/edge)
  2. the user can scroll but it will close the drop down automatically

As I have to dev an interface I was asking myself which behavior I should enforce.

Thanks,

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