generate 256-color palette #2283

Hi Foot team,

I believe that terminals should generate the extended 256-color palette based on the user's base16 theme.

The rationale and approach is written up here.

This PR introduces a new generate-256-palette config option, (削除) enabled by default (削除ここまで) opt-in, which generates colors 16-255 by interpolating the user's base16 theme in CIELAB color space. No colors explicitly defined by the user are replaced.

I have introduced this to a few terminals already and plan to offer it to more soon.

This PR is not ready:

There is active discussion at the Ghostty repo about approaching this slightly differently.

The idea is that light themes should not invert the palette so that ansi 16 remains black. This is with the intention of removing the breaking change of generating by default. Then, an opt-in option for "harmonious" colors which inverts the colors for light themes since there are advantages for doing so.

From the rationale doc:

The drawback is that 16 colors is limiting. Complex and color-heavy programs struggle with such a small palette.

Do you have evidence that this is really a problem? I can't say I have run into this myself.

Do you have evidence that this is really a problem? I can't say I have run into this myself.

I find subtle background colors lacking for layered diffs. Grey shades missing for different levels of UI (status lines, visual mode, popups).

This PR is tested and ready.

It adds two configuration options that are both opt-in:

• palette-generate Whether to generate the extended 256-color palette based on the user's base16 theme.
• palette-harmonious Whether to invert generated light themes for more semantic colors.

The config and behaviour is consistent with other PRs/emulators.

The default 256-color palette interpolates from black to white. This means it works better for dark themes since the dark shades are closer to the background and light shades are closer to the foreground. For light themes, the opposite is true, and so a UI designed for dark themes using the default 256-color palette looks bad with light themes.

That is the purpose of the palette-harmonious option: It inverts light themes so light/dark palettes are semantically equivalent. It makes shades interpolate from BG to FG instead of from black to white.

The palette-harmonious is opt-in to be consistent with Ghostty. You could delete this option and only ever generate harmonious palettes. This would be fine since the palette generation is opt-in. However, you will be unable to make palette-generate default in the future without breaking changes.