Pretty sure most of you already know this but for those who don't: you have two clipboards in Linux. One is the traditional clipboard where you copy with control c and paste with control v. The other one is when you highlight text and use the mouse middle click to paste text.
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Yes. You can test this by selecting something, closing that window and attempting to paste. It won't work. Closing the window removes the information about what was highlighted, so there is nothing to paste. If it were to copy upon selection you'd still be able to paste.