fix: keep explorer selections readable on light terminals

tests/unit/tui.rs

@@ -32,6 +32,74 @@ impl Drop for Directory {
     }
 }
 
+#[test]
+fn explorer_selection_follows_light_background_and_restores_dark_styles() {
+    let directory = Directory::new();
+    directory.file("arguments.hon", "return 0\n");
+    let mut picker =
+        FilePicker::from_paths(directory.0.clone(), [(0, PathBuf::from("arguments.hon"))]);
+    let light = ThemeColors {
+        background: [255, 246, 230],
+        red: [177, 52, 37],
+        green: [82, 101, 31],
+    };
+    let dark = ThemeColors {
+        background: [31, 23, 20],
+        red: [224, 120, 100],
+        green: [169, 182, 101],
+    };
+    let mut views = BufferViewports::default();
+    let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(70, 12)).unwrap();
+    for theme in [Some(light), Some(dark), None, Some(light)] {
+        views.set_theme(theme);
+        for focused in [false, true] {
+            picker.focused = focused;
+            terminal
+                .draw(|frame| {
+                    draw_file_list(frame, &picker, frame.area(), "", &mut views.panels, None);
+                })
+                .unwrap();
+            let buffer = terminal.backend().buffer();
+            let cell = buffer
+                .content
+                .iter()
+                .find(|cell| cell.symbol() == "a" && cell.modifier.contains(Modifier::BOLD))
+                .expect("selected arguments.hon row");
+            if theme == Some(light) {
+                let Color::Rgb(r, g, b) = cell.bg else {
+                    panic!("light selection must not use a dark ANSI background");
+                };
+                let luminance = |rgb: [u8; 3]| {
+                    rgb.into_iter()
+                        .zip([0.2126, 0.7152, 0.0722])
+                        .map(|(c, w)| {
+                            let c = f64::from(c) / 255.0;
+                            w * if c <= 0.04045 {
+                                c / 12.92
+                            } else {
+                                ((c + 0.055) / 1.055).powf(2.4)
+                            }
+                        })
+                        .sum::<f64>()
+                };
+                for foreground in [[69, 49, 38], [138, 98, 28], [153, 91, 43]] {
+                    assert!((luminance([r, g, b]) + 0.05) / (luminance(foreground) + 0.05) >= 4.5);
+                }
+                assert_ne!([r, g, b], light.background, "selection must remain visible");
+            } else {
+                assert_eq!(
+                    cell.bg,
+                    if focused {
+                        Color::Blue
+                    } else {
+                        Color::DarkGray
+                    }
+                );
+            }
+        }
+    }
+}
+
 #[allow(clippy::too_many_arguments)]
 fn rendered_whitespace_line(
     source: &str,