Theme: the field test only counts a fill or outline that is actually painted (#1204)

crates/ui-egui/src/field_frame_tests.rs

@@ -5,7 +5,7 @@
 use deckcraft_engine::Session;
 use egui::accesskit::Role;
 use egui::epaint::Shape;
-use egui::{Color32, Rect};
+use egui::{Color32, Rect, Stroke};
 use egui_kittest::Harness;
 use egui_kittest::kittest::Queryable;
 use serde_json::json;
@@ -31,7 +31,7 @@ fn distance(a: Color32, b: Color32) -> u8 {
 }
 
 /// The frame egui painted for the text field at `field` (its fill and outline).
-fn frame_at(h: &Harness<'_, SlideApp>, field: Rect) -> (Color32, egui::Stroke) {
+fn frame_at(h: &Harness<'_, SlideApp>, field: Rect) -> (Color32, Stroke) {
     h.output()
         .shapes
         .iter()
@@ -42,20 +42,49 @@ fn frame_at(h: &Harness<'_, SlideApp>, field: Rect) -> (Color32, egui::Stroke) {
         .unwrap_or_else(|| panic!("no frame painted for the field at {field:?}"))
 }
 
+/// What `c` shows when painted over the opaque `surface`. `Color32` is premultiplied, so each
+/// channel is `c + surface * (255 - alpha) / 255`.
+fn over(c: Color32, surface: Color32) -> Color32 {
+    let behind = |s: u8| (u16::from(s) * u16::from(255 - c.a()) / 255) as u8;
+    Color32::from_rgb(c.r().saturating_add(behind(surface.r())), c.g().saturating_add(behind(surface.g())), c.b().saturating_add(behind(surface.b())))
+}
+
+/// A frame with this `fill` and `stroke` stands apart from the opaque `surface`: it paints a fill
+/// or an outline (at least 1 px wide) that, as actually shown over the surface, differs from it.
+fn stands_apart(fill: Color32, stroke: Stroke, surface: Color32) -> bool {
+    let outlined = stroke.width >= 1.0 && distance(over(stroke.color, surface), surface) >= 24;
+    outlined || distance(over(fill, surface), surface) >= 16
+}
+
 /// The field stands apart from `surface` by its fill or by its outline.
 fn assert_visible(h: &Harness<'_, SlideApp>, what: &str, surface: Color32) {
     let fields: Vec<Rect> = h.query_all_by_role(Role::TextInput).map(|n| n.rect()).collect();
     assert!(!fields.is_empty(), "{what}: no text field");
     for field in fields {
         let (fill, stroke) = frame_at(h, field);
-        let outlined = stroke.width >= 1.0 && distance(stroke.color, surface) >= 24;
         assert!(
-            outlined || distance(fill, surface) >= 16,
+            stands_apart(fill, stroke, surface),
             "{what}: the field at {field:?} (fill {fill:?}, outline {stroke:?}) can't be told from the {surface:?} behind it"
         );
     }
 }
 
+#[test]
+fn only_a_painted_fill_or_outline_stands_apart() {
+    let surface = Color32::WHITE;
+    // Stroke::NONE is transparent black: nothing is painted, whatever its RGB.
+    assert!(!stands_apart(surface, Stroke::NONE, surface));
+    assert!(!stands_apart(Color32::WHITE, Stroke::NONE, Color32::from_rgb(250, 250, 250)));
+    // A fully transparent fill shows the surface.
+    assert!(!stands_apart(Color32::TRANSPARENT, Stroke::NONE, surface));
+    assert!(!stands_apart(Color32::TRANSPARENT, Stroke::new(0.5, Color32::BLACK), surface));
+    // A transparent outline paints nothing either.
+    assert!(!stands_apart(surface, Stroke::new(1.0, Color32::TRANSPARENT), surface));
+    // A real 1 px dark outline, or a clearly different opaque fill, does.
+    assert!(stands_apart(surface, Stroke::new(1.0, Color32::from_gray(120)), surface));
+    assert!(stands_apart(Color32::from_gray(200), Stroke::NONE, surface));
+}
+
 #[test]
 fn light_text_fields_show_in_dialogs_panes_and_the_ribbon() {
     let light = theme::Tokens::light();
@@ -71,7 +100,9 @@ fn light_text_fields_show_in_dialogs_panes_and_the_ribbon() {
     h.run_steps(2);
     assert!(h.ctx.memory(|m| m.focused().is_some()), "the click puts the keyboard in the field");
     let (fill, stroke) = frame_at(&h, field);
-    assert!(distance(fill, light.card) >= 16 || distance(stroke.color, light.card) >= 24, "focused field: fill {fill:?}, outline {stroke:?}");
+    // Same surface as the dialog check above: the field sits on the dialog's window fill.
+    let surface = h.ctx.global_style().visuals.window_fill;
+    assert!(stands_apart(fill, stroke, surface), "focused field: fill {fill:?}, outline {stroke:?}");
 
     // Review › Thesaurus: the search box on the task pane.
     let mut h = harness(SlideApp::new(Session::with_new(), Services::default()));