A weird UI thing in Windows 8's Explorer

I'm currently in the process of generating a huge file and blasting it across the network to a Windows share. On the machine at which the file is appearing, I sometimes press F5 on the Explorer window to see how much has transferred so far. I noticed while madly mashing that key that if I press it a few times really fast and then hold it for a few seconds, the items under "Select" in the Home tab of the ribbon will become disabled until I release the F5 key. The file size doesn't update continuously, though.