md-manuscript/src/app/style.rs

//! Global egui styling: font scaling, list/editor contrast tuning and the
//! colour helpers they use.

use super::*;

/// Fraction each base font size is enlarged, applied once at startup so every
/// panel's text is slightly bigger without changing layout maths elsewhere.
pub(super) const FONT_SCALE: f32 = 1.12;

/// Configure the shared context style once at startup: raise the contrast of
/// *all* UI text (menus, top bar, buttons, panels, headings — everything, not
/// just the file list and editor) and enlarge every base font size slightly.
///
/// The default dark theme uses fairly dim greys (~140) for body text, the main
/// legibility complaint; we pull each widget state's foreground toward white in
/// dark mode / black in light mode. Explicitly coloured spans (grammar
/// underlines, the cheatsheet's syntax colour) set their own colours and are
/// unaffected. The per-pane boosts (`boost_list_contrast`, the editor contrast
/// slider) still layer on top for their specific widgets.
pub(super) fn apply_global_style(ctx: &egui::Context) {
    let mut style = (*ctx.style()).clone();

    // Enlarge every text style (Body, Button, Heading, Monospace, Small).
    for font in style.text_styles.values_mut() {
        font.size = (font.size * FONT_SCALE).round();
    }

    let v = &mut style.visuals;
    let target = if v.dark_mode {
        egui::Color32::WHITE
    } else {
        egui::Color32::BLACK
    };
    let w = &mut v.widgets;
    w.noninteractive.fg_stroke.color =
        lerp_color(w.noninteractive.fg_stroke.color, target, 0.55);
    w.inactive.fg_stroke.color = lerp_color(w.inactive.fg_stroke.color, target, 0.45);
    w.hovered.fg_stroke.color = lerp_color(w.hovered.fg_stroke.color, target, 0.35);
    w.active.fg_stroke.color = lerp_color(w.active.fg_stroke.color, target, 0.35);
    w.open.fg_stroke.color = lerp_color(w.open.fg_stroke.color, target, 0.45);

    ctx.set_style(style);
}

/// Raise the text contrast of list-style widgets in the current `ui`, so file
/// names in the left pane stay legible against the panel background. Only the
/// widget foreground colours are touched; layout and the rest of the app are
/// unaffected (the change is scoped to the panel that calls this).
pub(super) fn boost_list_contrast(visuals: &mut egui::Visuals) {
    if visuals.dark_mode {
        // Unselected rows, hover, and the pressed state → toward white.
        visuals.widgets.inactive.fg_stroke.color = egui::Color32::from_gray(235);
        visuals.widgets.hovered.fg_stroke.color = egui::Color32::WHITE;
        visuals.widgets.active.fg_stroke.color = egui::Color32::WHITE;
        // Selected-row text (SelectableLabel uses selection.stroke when selected).
        visuals.selection.stroke.color = egui::Color32::WHITE;
    } else {
        visuals.widgets.inactive.fg_stroke.color = egui::Color32::from_gray(20);
        visuals.widgets.hovered.fg_stroke.color = egui::Color32::BLACK;
        visuals.widgets.active.fg_stroke.color = egui::Color32::BLACK;
        visuals.selection.stroke.color = egui::Color32::WHITE;
    }
}

/// The editor's text colour, interpolated from the theme's default toward the
/// maximum-contrast colour (white in dark mode, black in light mode) by
/// `contrast` percent (0 = theme default, 100 = full contrast).
pub(super) fn editor_text_color(visuals: &egui::Visuals, contrast: f32) -> egui::Color32 {
    let base = visuals.text_color();
    let target = if visuals.dark_mode {
        egui::Color32::WHITE
    } else {
        egui::Color32::BLACK
    };
    lerp_color(base, target, contrast / 100.0)
}

/// Linearly interpolate between two colours in gamma (sRGB byte) space.
/// `t` is clamped to `[0, 1]`; `t = 0` yields `a`, `t = 1` yields `b`.
pub(super) fn lerp_color(a: egui::Color32, b: egui::Color32, t: f32) -> egui::Color32 {
    let t = t.clamp(0.0, 1.0);
    let mix = |x: u8, y: u8| (x as f32 + (y as f32 - x as f32) * t).round() as u8;
    egui::Color32::from_rgb(
        mix(a.r(), b.r()),
        mix(a.g(), b.g()),
        mix(a.b(), b.b()),
    )
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn lerp_color_hits_endpoints_and_midpoint() {
        let a = egui::Color32::from_rgb(0, 0, 0);
        let b = egui::Color32::from_rgb(200, 100, 50);
        assert_eq!(lerp_color(a, b, 0.0), a);
        assert_eq!(lerp_color(a, b, 1.0), b);
        assert_eq!(lerp_color(a, b, 0.5), egui::Color32::from_rgb(100, 50, 25));
        // t is clamped to [0, 1].
        assert_eq!(lerp_color(a, b, -1.0), a);
        assert_eq!(lerp_color(a, b, 2.0), b);
    }

    #[test]
    fn editor_contrast_moves_toward_white_in_dark_mode() {
        let dark = egui::Visuals::dark();
        // 0% keeps the theme's text colour; 100% is fully white.
        assert_eq!(editor_text_color(&dark, 0.0), dark.text_color());
        assert_eq!(editor_text_color(&dark, 100.0), egui::Color32::WHITE);
        // In light mode, full contrast is black instead.
        let light = egui::Visuals::light();
        assert_eq!(editor_text_color(&light, 100.0), egui::Color32::BLACK);
    }
}