Problem

You are using chrome and in the tab bar is an annoying arrow on the very left pointing down.

Solution Version 123

Go to Chrome Flags and search for "customize chrome side panel".
Disable that and relaunch.

Caveat

The name of the feature changes from time to time.
It once was "Chrome Refresh 2023" and is now "Customize Chrome Side Panel".
It might change again in the future rendering this here wrong.

Last time I found the new feature name in the youtube comments in this video.

Solution Version 125

Via MANUKARTOFANU and Reddit I found that the flag was still available but not shown in the flags list anymore. EHRVERA in this Mega Thread (r/chrome) describes the solution.

So you'll need to disable the flag via the properties of the shortcut* you are using to open the browser.
"C:\Program Files\Google\Chrome\Application\chrome.exe" --disable-features=CustomizeChromeSidePanel

*shortcut: use a shortcut. Not the .exe file itself.

Solution Version 126

Now you can't get rid of it anymore.
Really annoying.