Colour schemes: Foreground and background "black" are not the same in the terminal

[ghost]

@oskar-skog raises an interesting observation in #24955

The foreground and background "black" colour are not the same.

From their screenshot:

However, this does not happen in Base16 Dark colour scheme:

But it does happen in many other colour schemes, including for example, the default one, as well as Monokai:

Is this expected behaviour?

[nico]

In Terminal.app's built-in "Silver Aerogel", the text is visible too:

(In most themes, it isn't.)

So I suppose this isn't a reliable way to draw invisible text.

(In Serenity's Terminal, if we were to implement concealing by setting the foreground color to the background color, we'd set the foreground color to the actual background color, and this wouldn't be an issue. But it's a moot point now that we didn't implement it that way.)

[ghost]

Then I have to wonder: Why do so many terminal libraries implement concealed text this way?

It appears to me it wouldn't even be a matter of performance: My implementation completely ignores drawing characters, and should be much faster, and yet we see st and iTerm doing concealed text with colours :thinking: