According to developers.google.com the following is true in Chrome: Don't allow popups during page unload Pages may no longer use window.open() to open a new page during unload. The Chrome popup blocker already prohibited this, but now it is prohibited whether or not the popup blocker is enabled. Koha uses windows.open() to open print dialogues for Print Slip and Print Quick Slip. These no longer open as members-menu.js uses windows.open().
I think the important detail relevant to us is that this new restriction is for window.open() *during unload.* I would take that to mean that we can't trigger a pop-up window with the unload event (https://developer.mozilla.org/en-US/docs/Web/API/Window/unload_event). The only instance of "onunload" I see in the code is in SCO and it isn't used to open a window.
Owen, You are correct, I filed this bug to hastily. Marking it resolved now.