Private message window>Close Window link doesn't close the window; no error messages

I sent a private message to another member.  Clicking the button to send a message opened a second browser window/tab to present the new message editor.  I entered text, sent the message successfully.

I clicked the Close Window link, to close this tab, and nothing happened.

When I moused over the link, my cursor changed to indicate that the link was active and could be clicked.  But clicking it had no visible effect.  No errors in the UI, and also, there were no errors recorded in the developer tools>Console or Network tabs.

I just closed the browser tab by clicking its X button, but the link should work, and could indicate something deeper.

Low severity.