I am guessing that N++ or its GUI framework uses a transparent overlay.  For tkinter, that would mean a transparent Canvas on which one could draw vertical dotted gray lines 1-pixel wide.  However, Serhiy Storchaka in msg213643 of #20920 said: "Tk supports alpha only for photo images and as an attribute of top-level window. It doesn't support alpha component in colors."

I searched 'tkinter transparent canvas'.

For pixel positioning, photos with transparent backgrounds can only be used on a canvas.  This was one of the answers to https://stackoverflow.com/questions/53021603/how-to-make-a-tkinter-canvas-background-transparent
More details, using pil(low) to make images, are in the following:
https://www.tutorialspoint.com/how-to-make-a-tkinter-canvas-rectangle-transparent
https://www.javaer101.com/en/article/921105.html

Another answer to the SO question, for Windows only, was to pip install pywin32 and use various calls to make a canvas layered with a transparent colorkey.  Perhaps the same could be done, at least on Windows, with ctypes.

A top-level background is made (partially) transparent with "top.attributes('-alpha', d)" where d in [0.0-1.0].  On Windows, 'top.wm_attributes('-transparentcolor', keycolor) makes that keycolor actually be transparent, like the green/blue screen used in television and movies. But any keycolor pixel in the toplevel exposes the screen beneath the toplevel, not the widget in the toplevel.  This detail is not clear in the tk docs.
https://www.youtube.com/watch?v=75jbNpc8vN4
What also is not clear is whether the keycolor applies only to the one toplevel or all toplevels, nor what happens when one toplevel is over another.

I summarized the above so I or anyone else can find the information.  I am closing this issue for now, but someone can reopen if the situation changes.