Using the `mouseDownOutside` event was problematic here because two
events were being triggered consecutively: `mouseDown`
would toggle the menu off and `click` would then toggle it back on. This
switches the logic to use `clickOutside` again, but with two changes:
- it limits the action to the `search-menu` key (so that theme component
overrides can do their own handling)
- it does not trigger the event when there is an active text selection
(this was the original reason for switching to `mouseDownOutside`, see
https://github.com/discourse/discourse/pull/14788)
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