It seems that QEMU/KVM's default PS2 mouse and keyboard doesn't work right in most cases. I have especially observed issues using Ctrl+C and Ctrl+V and in Linux you may see repeated key presses in the terminal and you will wonder why you copied something and it's not in the clipboard when you try to paste. The way to temporarily fix it is to press the key that is repeating once (works in Linux but not really in Windows).
Sometimes when moving your mouse it will also select everything on the screen or click something on it's own (possible click repetition similar to the repeating key issue).
The way around this is to NEVER use the default QEMU PS2 mouse and keyboard (if you run qemu-system-x86_64 without specifying a USB keyboard and mouse, it gives you the bad / buggy PS2 by default).
When starting QEMU just pass these flags to give yourself a USB keyboard and mouse:
-usb -device usb-mouse -device usb-kbd -device usb-tablet
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