The behaviour I am suggesting is the same of the native show "symbol references" visual studio popup window. (I am referring to that popup that opens when you click on the "<n> references" tag visual studio shows on top of method and properties declarations)

it is a pop up, but in its top-right corner there is a icon that, when clicked, turns it in a docked tab in the editor that does not go away.

the current approach (a simple popup window that goes away as soon as you start interacting with the editor) makes this list of very little use: if you want to check the source code of all the tests covering such line, you have every time to go back to such line (if you remember where it was), reopen the popup and remember which was in the list the last test you checked.