It is likely your Mac is in "dual" display mode. In this mode you have your laptop display and an "alternate" screen to the right, left, above or below the laptop display. If you move some open windows to each "edge" of the display, you may see part the moved window.
To "fix" this, you probably want to "Turn on mirroring". To do this, open System Preferences and click the "Displays" icon. Choose the Arrangement tab - then click the box "Mirror Displays". Now the display will "mirror" what is on your screen.
I use this feature often switching between the mode of the class seeing exactly what is on my screen and the mode where the class sees one thing and I see something else (I can be reading email while they are answering a question).
If you can see anything from your Mac on the external display, it is not a dongle or GVA cable problem.