Not a Windows 10 user, but when using as an extender monitor, try changing the resolution within the windows settings. (Advanced display settings) when connected.
I have to sometimes do this on my laptop depending on which tv I am connecting to via HDMI. (Changing from 800x600 to 1920x1080, etc)
Basically shrinking the display screen to fit on the TV screen.
Others w/ Win10 knowledge should be able to give you better detailed info or suggestions if needed.
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