Go back to previous window config: sometimes impossible

[Kai Großjohann]

Open a file.  Move point to position A in that file.  Hit C-x 2.
Move point to position B in the new window.  Use C-h f or something
else that frobs the window configuration.

There is no way to go back to the previous situation!

(One of the positions in the file has been lost.)

I tried winner-mode, but it didn't remember two positions, either.
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