Strange "ghost images" / transparency effect in full-screen videos

A strange problem manifested on my machine recently, and I'm really at a loss when trying to resolve it, as I don't even know how to google this.

I was watching Netflix full-screen one fine day, on Firefox v64.0, on my Ubuntu 18.04.1 laptop. At some point, I thought I saw ghost images overlaid on my video. I thought my eyes were just tired and paid no attention to it, I closed the laptop and went to get some coffee.

Later on, while watching a YouTube video (again, fullscreen) and I thought I saw the ghost image again. I turned up my screen's brightness all the way to 100% and lo-and-behold, I could see my wallpaper through the full-screen video!

This has been going on for some days now, and I wasn't able to find what's causing it. Full screen videos on Netflix and YouTube have some sort of 10-20% transparency, and I can see my wallpaper behind them. This only happens when the video is playing. When I pause the video, transparency goes away. You can understand how this played with my mind a little bit at first.

Local video files on my machine, played through mpv, have no transparency issues.

To give you an example, I'm sorry but I literally took a photo of my laptop's monitor.

Here's a full screen YouTube video of a pure black screen, for 10 hours (It's just a video of all-black playback):

The image in the back (the ghost image I was seeing) is of course, nothing else other than my wallpaper:

The transparency is also captured in regular screenshots. Here's one of the above 100%-black-video fullscreened and playing. It isn't as visible because it's a software screenshot, but you can see the ghost image there too if you squint.