Vertical sub-menu does not close

Configuration:
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Home page vertical menu enabled on the left with sub-menu items.

Scenario:
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Configure a vertical sub-menu url to refer to a pdf file on a different server and enable the “open link in a new tab” checkbox.

Problem Trigger:
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Click on a sub-menu to trigger the creation of a new browser tab displaying the referenced pdf file. Return to browser tab for the webpage the sub-menu item is on.

Problem:
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Sub-menu group is still open even though mouse focus moved / moves out of the sub-menu group screen area.

Confusingly, if you then move your mouse over the vertical main menu items, their respective sub-menu groups open and close in a normal fashion but the “stuck-open” sub-menu group remains stuck open even if reselected on the main menu.

It is as if the sub-menu group is stuck waiting for a “close” event that never arrives.

Work-around:
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If your users know enough to click on a blank area of the page outside the menu area then the menu / sub-menu starts to work correctly again.

Anyone have any thoughts as to what “close” event is getting lost in this use-case and how to corect?

Thanks in advance.