Many thanks @martinschmelzer!

I've improved the implementation a bit, see mtennekes@ebe88f8 Instead of maxWidth, I opted for width which is more general (1 means max width). Let me know if it works as expected.

You are very welcome. I'll try it out tomorrow.

@mtennekes Take the following baseic example:

data(World, metro, rivers)

tm_shape(World, projection=4326) + 
  tm_polygons() + 
  tm_credits("Test", width = 1, bg.color = "red", position = c(0,0)) +
  tm_layout(frame = T, inner.margins = c(0,0,0,0))

The credits are usually not positioned in (0,0). When I do so and set width = 1 while also having no inner.margin, the credits do not span the whole map width. Is that the desired behavior? I initially thought inner.margins = c(0,0,0,0) would lead to full span credits.

Found out that it is not the inner.margins, but other types of margins. I agree that width = 1 should correspond to the full map width.
So I implemented it that way now.