Automatically adjust ANSI color contrast (#34033)

Closes #33253 in a way that doesn't regress #32175 - namely,
automatically adjusts the contrast between the foreground and background
text in the terminal such that it's above a certain threshold. The
threshold is configurable in settings, and can be set to 0 to turn off
this feature and use exactly the colors the theme specifies even if they
are illegible.

## One Light Theme Before
<img width="220" alt="Screenshot 2025-07-07 at 6 00 47 PM"
src="https://github.com/user-attachments/assets/096754a6-f79f-4fea-a86e-cb7b8ff45d60"
/>

(Last row is highlighted because otherwise the text is unreadable; the
foreground and background are the same color.)

## One Light Theme After

(This is with the new default contrast adjustment setting.)

<img width="215" alt="Screenshot 2025-07-07 at 6 22 02 PM"
src="https://github.com/user-attachments/assets/b082fefe-76f5-4231-b704-ff387983a3cb"
/>

This approach was inspired by @mitchellh's use of automatic contrast
adjustment in [Ghostty](https://ghostty.org/) - thanks, Mitchell! The
main difference is that we're using APCA's formula instead of WCAG for
[these
reasons](https://khan-tw.medium.com/wcag2-are-you-still-using-it-ui-contrast-visibility-standard-readability-contrast-f34eb73e89ee).

Release Notes:

- Added automatic dynamic contrast adjustment for terminal foreground
and background colors
This commit is contained in:
Richard Feldman 2025-07-07 18:39:11 -04:00 committed by GitHub
parent 877ef5e1b1
commit 9b7632d5f6
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6 changed files with 669 additions and 8 deletions

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@ -1352,7 +1352,7 @@
// 5. Never show the scrollbar:
// "never"
"show": null
}
},
// Set the terminal's font size. If this option is not included,
// the terminal will default to matching the buffer's font size.
// "font_size": 15,
@ -1369,6 +1369,21 @@
// Default: 10_000, maximum: 100_000 (all bigger values set will be treated as 100_000), 0 disables the scrolling.
// Existing terminals will not pick up this change until they are recreated.
// "max_scroll_history_lines": 10000,
// The minimum APCA perceptual contrast between foreground and background colors.
// APCA (Accessible Perceptual Contrast Algorithm) is more accurate than WCAG 2.x,
// especially for dark mode. Values range from 0 to 106.
//
// Based on APCA Readability Criterion (ARC) Bronze Simple Mode:
// https://readtech.org/ARC/tests/bronze-simple-mode/
// - 0: No contrast adjustment
// - 45: Minimum for large fluent text (36px+)
// - 60: Minimum for other content text
// - 75: Minimum for body text
// - 90: Preferred for body text
//
// Most terminal themes have APCA values of 40-70.
// A value of 45 preserves colorful themes while ensuring legibility.
"minimum_contrast": 45
},
"code_actions_on_format": {},
// Settings related to running tasks.