YouTube full screen not covering entire TV screen

My TV is hooked up to my PC via HDMI, but I've run into a weird glitch. When I switch a YouTube video to full screen, it doesn't actually fill the display—I end up with these desktop borders visible at the top and left.
Everything looks perfectly fine when the browser is just running in its standard windowed mode.
I already messed around with the overscan and underscan settings, but nothing seems to budge. I'm running Windows 7.
It’s not just YouTube either; the same thing happens with any other streaming service I open in the browser.
The strange part? If I play a movie through a dedicated media player, it fills the entire screen without a problem.
This issue is exclusive to the TV; my computer monitor handles everything perfectly.

Does anyone know how to fix this?

So, how are you actually sending the image to the TV? Are you going with second screen only, extended, or just duplicating the display? If I were you, I'd try "second screen only." Duplicating can be a total headache when your monitor and the TV have different aspect ratios. It just gets messy.

I'm using an extended display setup. The monitor is 1080p, but the TV is only 720p.

It works fine on Firefox. This issue is happening on Chrome.