header unexpected crop

Hello! I am new user at WP. I have chosen theme Twenty Seventeen for developing my first website. Thanks for support in this forum 🙂
I am quite close to complete and publish site but I have some unexpected problems to solve. The most “critical” is the appereance of header. I created my own animated gif 2000×1200 px, converted into mp4 (about 7MB) and loaded as header. I also added one picture (same resolution 2000×1200) as “static” picture in case of problems during visualization.
Now, despite of I kept suggested default resolution when I visualize my site on notebook (resolution 1366×768) I get it cropped at the top and at the bottom. about 140 px at the top and 240 px at the bottom. So the visibile gif has became about 2000×820 px.
That’s quite “annoying, above all because the video has sense if you can watch it at full resolution.
When I check the previews available during modification (the three options for PC, tablet and smartphone)…it looks ok…
What’s problem? resolution of notebook display or other?

Thanks