if you open more files in the ScriptEditor than fit in the file tab window size, you can do this, but you cannot access these files directly. You have to resize the window or close files on the left side of the window.
In other editors you can find different solutions, either you find right left file buttons or drop down lists to the select files that are outside the current window width.
hi @daniel.kowollik you can browse through the tabs with the scroll wheel on your mouse. But having a list in Window makes sense. RH-84935 Access list of open documents