Identifiy this Visual Studio sign, please
I must have pressed a wrong keyboard combo and got this little arrow sign in my Visual Studio 2019, left of the code, where breakpoints reside. Could not find any way to identify it. Google image search and Bing Visual Search could not find anything.
So, what is this, please? 
Ok, a broader view in context: 
Restarting VS made is disappear, but now I am so curious, what feature is this?
Solution
This is a task list shortcut. You can toggle it with Ctrl+K, Ctrl+H. You can open a task list window with Ctrl+\, T
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