Don't close issue sidebar label select box on click if only mouseup outside

What does this MR do?

Issue sidebar > label selection: don't collapse auto suggest label select box on click if the corresponding mousedown event has happened inside the box, see #33748 (closed).
This MR is a follow-up to !51139 (merged) (which already fixed the same problem for Bootstrap Dropdown based select boxes).
The component affected by this MR is a custom Vue component, therefore it was not fixed with !51139 (merged).

Does this MR meet the acceptance criteria?

Conformity

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