Child-frame can not move to proper position when run "env GDK_SCALE=2 emacs"

[Feng Shu]

When I run "env GDK_SCALE=2 emacs", the below code can not work properly

```
(let ((test (let ((after-make-frame-functions nil))
              (make-frame
               `((parent-frame . ,(window-frame))
                 (width . 50)
                 (height . 1)))))
      (pos (posn-x-y (posn-at-point (point) (selected-window)))))
  (set-frame-position test (car pos) (cdr pos)))<I>
```

When I eval the code at the <I>, the new created frame can not move to
<I>, it move to the wrong position.




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