Tests: the tool tip's box keeps the theme's fill and contrast

The test for #3818 checked the tool tip's text color and the style's
window fill. It now also checks the fill of the box drawn behind the tool
tip's text, and that the text and the box have a contrast ratio of at
least 4.5, for every brightness and scheme.

crates/ui-egui/src/theme.rs

@@ -705,19 +705,50 @@ mod tests {
                             ui.button("Hover me").on_hover_text("Tip text");
                         });
                         out.textures_delta.clear();
-                        tip = out.shapes.iter().find_map(|c| match &c.shape {
-                            egui::Shape::Text(s) if s.galley.text() == "Tip text" => Some(s.fallback_color),
+                        // The tool tip's text color, and the fill of the box drawn behind it.
+                        let mut shapes: Vec<&egui::Shape> = Vec::new();
+                        let mut open: Vec<&egui::Shape> = out.shapes.iter().rev().map(|c| &c.shape).collect();
+                        while let Some(shape) = open.pop() {
+                            match shape {
+                                egui::Shape::Vec(inner) => open.extend(inner.iter().rev()),
+                                shape => shapes.push(shape),
+                            }
+                        }
+                        tip = shapes.iter().enumerate().find_map(|(n, shape)| match shape {
+                            egui::Shape::Text(s) if s.galley.text() == "Tip text" => {
+                                let fill = shapes[..n].iter().rev().find_map(|behind| match behind {
+                                    egui::Shape::Rect(r) if r.rect.contains(s.pos) && r.fill.a() > 0 => Some(r.fill),
+                                    _ => None,
+                                });
+                                Some((s.fallback_color, fill))
+                            }
                             _ => None,
                         });
                     }
                     let v = ctx.global_style().visuals.clone();
-                    assert_eq!((v.window_fill, v.override_text_color), (t.panel, Some(t.text)), "{scheme:?} {b:?}, system {first:?} then {then:?}");
-                    assert_eq!(tip, Some(t.text), "{scheme:?} {b:?}, system {first:?} then {then:?}: the tool tip's text");
+                    let what = format!("{scheme:?} {b:?}, system {first:?} then {then:?}");
+                    assert_eq!((v.window_fill, v.override_text_color), (t.panel, Some(t.text)), "{what}");
+                    assert_eq!(tip, Some((t.text, Some(t.panel))), "{what}: the tool tip's text and box");
+                    assert!(contrast(t.text, t.panel) >= 4.5, "{what}: the tool tip's text against its box, {}", contrast(t.text, t.panel));
                 }
             }
         }
     }
 
+    /// The contrast ratio of two opaque colors, from 1 (the same) to 21 (black on white), as WCAG 2
+    /// defines it.
+    fn contrast(a: egui::Color32, b: egui::Color32) -> f32 {
+        let luminance = |c: egui::Color32| {
+            let channel = |v: u8| {
+                let v = f32::from(v) / 255.0;
+                if v <= 0.040_45 { v / 12.92 } else { ((v + 0.055) / 1.055).powf(2.4) }
+            };
+            0.2126 * channel(c.r()) + 0.7152 * channel(c.g()) + 0.0722 * channel(c.b())
+        };
+        let (a, b) = (luminance(a), luminance(b));
+        (a.max(b) + 0.05) / (a.min(b) + 0.05)
+    }
+
     /// Changing brightness keeps the interface settings made after the theme was applied, such as
     /// General › Show Tool Tips' delay.
     #[test]