"When you see this checkmark, it means that the file, or files that have the checkmark on it are synchronized with Microsoft OneDrive. You can get rid of these by exiting OneDrive from the system tray."
Microsoft (wrong move IMO) continues to push OneDrive and it's not something I use but on big updates it reinstalls, rearms and this happens.
As to the WinKey. https://www.thewindowsclub.com/windows-key-winkey-not-working covers the usual reasons.
It seems like, but I'm not sure, WIN did an upgrade and I lost the pulldown of the WIN 10 Apps when I press the WIN Key. I also started seeing Green check marks on all the PDF's displayed on the desktop and I am not using Norton AV.
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