When adjusting the sound of YouTube video, a gray bar appears at the top of the screen

That bar is the SOUND LEVEL INDICATOR. When you increase or decrease the sound levels in the YouTube app then that bar appears to show you the current sound level. If you look closely to the left end of the bar then you will see the speaker icon.

Axel F.

Sparklet Said:

"When adjusting the sound of YouTube video, a gray bar appears at the top of the screen"

-------

About the Bar:

That is the measurement of the volume (shown by percentage in length). So, all seems fine, to me.

Your Image in Full: https://discussions.apple.com/content/attachment/89d7a4db-94ce-4b84-83dc-b2923a9b5a46

The Bar Maximized:

Sparklet Said:

"When adjusting the sound of YouTube video, a gray bar appears at the top of the screen: I mean this is problem on my iPhone, i don't have Mac"

-------

Two Thoughts:

A. Temporary Internet Files:

Try emptying the Temporary Internet Files of Safari

1. Go To: Settings
2. Tap: Safari
3. Select: Clear History and Website Data
4. Tap: Clear History and Data

Screenshot:

B. Try a different Browser:

See what happens when you view these videos on Web browsers other than Safari (i.e. Firefox or Chrome). Any difference?

Sparklet Said:

"When adjusting the sound of YouTube video, a gray bar appears at the top of the screen: This only happens in the YouTube app"

-------

What to Try:

So its the YouTube app? Try the following, in the order provided:

1. Delete: the YouTube app
2. Restart: the iPhone***
3. Re-download: the YouTube app
4. Restart: the iPhone
5. Try: the YouTube app

*** Performing a Manual Restart:

It might take. a couple of tries.

1. Go to: Settings
2. Tap: General
3. Scroll:  to bottom
4. Tap: Shut Down
5. Slide: finger to the right
6. Power it back on. Does it work?

Sparklet Said:

"When adjusting the sound of YouTube video, a gray bar appears at the top of the screen: Increased contrast, now you see?"

-------

Provide a Photo:

No. Nothing stands out, as peculiar nor anything that looks speculative. But, take a photo of this, add it as an attachment to a reply, and maybe something will be seen.

Thank you for the screenshot.

Now, by comparison, I see it.

Try a New Admin User:

So, create a new administrator (for temporary troubleshooting), and see if it is seen. If it is not seen, then it is consolidated to this current user. If it is seen, it is likely something misconfigured on your installed macOS.

Sparklet Said:

"No it's not working"

-------

One last thought comes to mind:

1. Sign Out: of iCloud
2. Restart: your iPhone
3. Sign In: to iCloud

Perhaps things need to resync.

Where are using YouTube? The App or via Safari?

I opened the screenshot in full screen and all I see if the volume slider bar, with the speaker icon on the left.