I am having an issue with Manjaro GNOME Keyboard shotcuts & smthn else

Hello. I am new to Linux and am using Manjaro GNOME.

My F8 key causes this weird sign to appear on my screen
which prevents me from using F8 in other applications.
How do I remove that?

In addition, I have an issue with dosBOX.
I am using dosBOX for school and am using TurboDebugger. When I press on the TD application in DBOX in locks my keyboard and mouse to that window which I cannot escape. Is there a fic to that too?