This situation happens a lot than you think, and on many web browsers. When you click to play Youtube or any websites in fullscreen mode, you can’t see the whole page because Windows’ taskbar stays on top of the web browser.
When you encounter this situation, consider using one of the following solutions. The 2nd solution is what helped me.
Check Web Browser
In some situations, it is the browser that leads to this problem. If you have a 2nd browser, open a Youtube video in fullscreen to see if it has the same issue. If it is not, re-install the browser will help.
Sometimes, disabling display scaling on high DPI settings on Chrome can help.
Restart Windows Explorer
Firstly, open your Task Manager by right-clicking on Taskbar then choose Task Manager in the popup menu.
On the Process tab, pull down all the way until you see Windows Explorer. Right-click on it then choose Restart.
Restart Your Computer
If none works, it is better to restart your computer to make every applications restart their functions.