This idea comes from madsrh's mockup.

It requires background-images for control buttons, so it's not a high priority task
But no circle on hover for min and max, right?
No circles, correct
I did some testing with this, but I decided to omit the boxed effect. It doesn't work without the white circle present in the mockup. Since the current buttons themselves rely on a circle effect when interacted with you end up with this double background color that looks like an error, too.
Should we merge the changes or try with another design instead?
Can you post a picture of the result?
Sure.
Background-image lets us use larger buttons without the background-color taking up the full dimensions of the box which would look awkward:
(Result of the pull request)

With the boxed effect, a 1:1 ratio would make the buttons too wide and the close button can't reach the edge of the headerbar unless we remove the headerbar's padding:

What do you think of removing the circle hover from the second picture?
I thought about it. We could make them look and work like regular headerbar buttons, but then the orange circle will need to go.
I think I might reduce the target space since it's too probably a little unreasonably large the way it is now. 2-3px outside of the circle's radius should be fair.
If I understood correctly, the effect should be this one

@godlyranchdressing do you mind if I commit this?
Of course, though I think it'd be better if they have a radius to match the other headerbar buttons.
Of course, though I think it'd be better if they have a radius to match the other headerbar buttons.
That'd be surely better
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If I understood correctly, the effect should be this one