@DavidPostill: Hi, 1/ Yes, the bug even occurs after restarting the computer. 2/ Yes, I cleared the Chrome cache also. The bug still occurs.

@DavidPostill: I started Chrome after deleting my Default profile, then I opened some YouTube tabs, and I still got the popup. (Sorry for the late answer)

What do you see under chrome://policy?

Run Speed Tracer and post the results here

@Vinayak: chrome://policy shows no rule.

Also try the Profiler. Check what is hogging the CPU by sorting results by total CPU usage. If you see a script taking up most of the resources, find out the source of it by clicking on it and then selecting 'Reveal in navigator'. See this for a demo

@Vinayak: I added the Speed Tracer and the Profiler results on my post. Thanks for the demo by the way.

It looks like you do have a few extensions installed. bfbmjmiodbnnpllbbbfblcpljjjepjdn is Turn Off the Lights.

@Vinayak: I installed Turn Off the Lights a very long time ago and it never bothered me. I didn't install a new extension very recently. Moreover, I started Chrome with a new user profile, without any extension installed, and I still got this bug.

Try deleting your user profile folder and then starting Chrome by running this command: rmdir /S /Q "%localappdata%\Google\Chrome\User Data\Default". Make sure Chrome is closed when you run it. It will delete all your cookies, history, extensions, bookmarks, saved passwords, etc. If you haven't backed all of that using Chrome Sync do so before you run the command. You can check if your data is already synced by checking Chrome Sync settings

@Vinayak: I have already done it, that's what I meant when I said I deleted my Default user profile. The problem persists.

I just renamed it and launched Chrome, which created a new Default folder

I see. And did you visit YouTube without signing in to Chrome after you renamed the folder and launched Chrome?

Yes, I did not sign to Chrome, nor to my Google Account.

I see. Do you have the latest version of Chrome installed? Do you face the same problem when using Portable Google Chrome?

I have the latest version : Version 39.0.2171.71 m

I will try the portable version

Okay, let me know how it goes