Expected behavior of the wanted feature

I'd like that MPV's sub-back-color looked like MPC-HC's opaque box, where the text of a shorter line is precisely covered by the back color, without the need of the box being a perfect rectangle:

On MPV on the other hand, it currently looks like this:

The sub-back-color feature is important to me because I'm visually impaired and use it to improve subtitles' readability, but the way it looks like on MPV is not the ideal I suppose.
Btw, I don't want the current behavior to be completely withdrawn, just that an option to make this behavior customizable existed (sub-back-precise=yes maybe? I can't think of a better name at the moment).

Alternative behavior of the wanted feature

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Log File

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Sample Files

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