Window moving when exporting markdown to a pdf file that has already exist.

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This is not a big issue, just a user-friendly-related one.
I'm using macdown on Macbook Pro 13' Retina.
When the macdown window is not maximized and is put on the left side of the screen(I did this because I was editing the markdown file while reading something else at the same time), I try to export it to a pdf file. By the time I export it to pdf from the menu, the window does not stay at where it was. Instead, it moves right a bit. If I choose to save the pdf file using a new name, the window will go back to the original position(left margin of the window). However, if I choose to use an existed name and replace the existed file, the window will not go back.

Reproduce the export-to-PDF flow on macOS with the window positioned on the left, comparing a new filename with overwriting an existing file. Trace the export and save-overwrite path; done means the window remains in its original position in both cases.

Written by the indexing model from the issue text.